@Benswan Welcome to the Community Ben, and thank you for subscribing and working through the theory course.
You have been given a good answer by @Jeff - it is all to do with enharmonic equivalent names and the actual, practical choices between those names that get used in musical settings.
Here is the Circle of Fifths in the lesson.
Here is a more fully labelled Circle of Fifths.
Most of the notes have two names. These are the enharmonic equivalent names.
Only three have a single name. Your first question may be why those three are given only one name. They do have enharmonic equivalent names but those would involve double sharps or double flats).
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G = F## or G = A♭♭
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D = C## or D = E♭♭
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A = G## or A = B♭♭
In addressing your question fully, I will take the various options a piece at a time.
First, the E and B labels. Why are these not commonly named F♭ and C♭. The first, most obvious answer, is that B and E are natural notes. A second answer would be that no musician would opt to name those notes using the enharmonic equivalents. The impracticality is obvious when you take each as the root of their major scales.
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E major scale = E, F#, G#, A, B, C#, D#
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F♭ major scale = F♭, G♭, A♭, B♭♭, C♭, D♭, E♭
It contains a double flat - yuck! It is simply best avoided given the alternative.
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B major scale = B, C#, D#, E, F#, G#, A#
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C♭ major scale = C♭, D♭, E♭. F♭, G♭, A♭, B♭
A little weird - it is all flats. Not so dreadful but simply not used as an option over the enharmonic equivalent.
Second, the C and F labels. Similar to above, why would anyone not choose the natural notes? Comparing the major scales of each again.
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C major scale = C, D, E, F, G, A, B
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B# major = B#, C##, D##, E#, F##, G##, A##
Oh, good grief - NO! Give me C every day of the week.
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F major scale = F, G, A, B♭, C, D, E
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E# major scale = E#, F##, G##, A#, B#, C##, D##
Preposterous! E# is not the winner.
Third, most of the notes with a sharp and a flat label.
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Db major scale = D♭, E♭, F, G♭, A♭, B♭, C
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C# major scale = C#, D#, E#, F#, G#, A#, B#
Neither are exactly user-friendly but C# simply isn’t used.
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A♭ major scale = A♭, B♭, C, D♭, E♭, F, G
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G# major scale = G#, A#, B#, C#, D#, E#, F##
A♭ is preferred, it has several natural notes and G# has all sharps plus a double sharp.
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E♭ major scale = E♭, F, G, A♭, B♭, C, D
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D# major scale = D#, E#, F##, G#, A#, B#, C##
Give me E♭ and avoid the two double sharps.
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B♭ major scale = B♭, C, D, E♭, F, G, A
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A# major scale = A#, B#, C##, D#, E#, F##, G##
A# is just another horrible thing to contemplate using when there is an easier alternative from its enharmonic equivalent.
Fourth, the remaining pair of F# and G♭
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F# major scale = F#, G#, A#, B, C#, D#, E#
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G♭ major scale = G♭, A♭, B♭, C♭, D♭, E♭, F
There is little to choose between them. Each have one natural note and six accidentals, either # or ♭. Many diagrams of a Circle of Fifths present these with both labels for this reason. Justin does it in the lesson (diagram above).
I hope that helps.
Cheers 
| Richard | JustinGuitar Approved Teacher, Official Guide & Moderator