@SeriousJuJuSo what we concluded is the XP requirement is tied to the amount of times you have completed the XP mission. So if you are on Level 87.0, completed 5 weeks of weeklies (24.5), got 5 free levels (5.0), and bought 6 (6.0), you have completed 51.5 levels via the XP mission.
I am at Level 85.0, did exactly the same but didn't buy levels. So I have completed 55.5 levels via XP mission, thus why my requirement is higher.
The XP requirement jumps up 500 every 1.5 levels. I am at 22K, you are at 20.5K, so you are 3 jumps away from me, or approximately 4.5 levels (using the XP mission), 55.5 - 51.5 = 4.0. That means you will flip to 21K in the next 0.6 levels when using the XP mission.
Also, I only remember getting 4 free items once I completed the first tier, not 5 levels free. So if that is the case, we are at 60.5 and 56.5 respectively. Either way, still 4 levels apart via the XP mission.