| Nobbs |
The math behind it follows the the rule for monster xp.
Rules
1) For slow/normal/fast advancement, you should level up after 7.5/5/3.25 encounters with a CR of your respective level
2) Two monsters with the same CR give together the same XP as one monster with CR+2. By this we defined the even and odd levels separately, but have not tied them together. Every two levels, you need the double amount of xp to advance, for level 2 -> 2000, lvl 4 -> 2*2000, lvl 6 -> 2*2*2000 etc.
3) To have nice numbers, they rounded the entries in the tables, both for level advancement and xp for an encounter. Therefore the used pattern does not hold strictly. Furthermore, instead of tying the even and odd levels together by a factor of 1.41..., they used 1.5 alternated by 1.3333.
Therefore the xp needed for a level is
1000 * sum(n=1;n=level)[1.41^n]
(which is a geometric progression), plus some rounding.