It makes a lot of sense. It just isn't intended to be linear.
If you take the "DEADLY ENCOUNTER" experience for a given level (for 1 PC), and divide it into the amount of experience you need for the next level, you get "deadlies / level".
To get to 2nd and 3rd, you need 3 deadlies / level (per PC).
4th is 4.5 deadlies, 5th is 7.6, then 6 to 10 is ~6.66 (with some small variation).
11 jumps up to 7.5, and it drops down to 4 to 4.5 per level from there on out.
In my experience, PCs can handle about 2.5 Deadlies per level in a 4 to 6 hour session. PCs do not just face deadly encounters, but generally speaking, easier ones go faster so the rate holds.
This means that PCs advance at around this pace:
1 or 2 sessions to 2nd level (for a 4 to 6 hour session).
1 or 2 session to 3.
2 session to 4.
3 sessions each for 5 to 10.
4 sessions for 11.
2 sessions per level from there on.
That rate allows PCs to go all the way through 20 levels in less than a year, if they're facing challenges at a pretty steady rate.
If your sessions are 3 hours, double the number of sessions.
And you want to know a secret that shows that this is intentional by the designers? Using these number of sessions per level, if you follow the guidance on treasure hoards per level, it splits up evenly to 1 treasure hoard per session (if you give no treasure hoard when they're 1st level adventurers).
7 hoards for levels 1 to 4 (which you'll get to in about 8 session)
18 hoards for levels 5 to 10 (which take 18 sessions)
12 hoards for 11 to 16 (which takes 12 sessions)
8 hoards for 17 to 20 (which takes 4 sessions).