There is no mistake as far as I know.
It's because the table is calculated and maintained by me (Ronga).
I do it to the best of my knowledge.That site simply stole the table without giving credits.
In fact the formula is not linear. It's rather complicated.
And yes, it adds +6 and then +5. Perhaps developer like the art of irregularities.
Well don't ever think of getting level 100. It's going to take years unless you do pupil spamming. Think about it. You have only max 6 experience per day. If you read the level table, each level requires more and more experiences.
The first 30 levels are more than enough for players who play the game fairly.