I was in a match (159696515) that had a player DC at level 1. When I used Tome at 26:56 in game (32:05 replay), it added very little extra XP.
After a bit of looking around, I believe the reason is that it's taking the sum of XP for all current players and dividing by 5, rather than the amount of remaining ones - in this case, 4. Here's why: the following table shows XP at that point. If you do all the math, it adds to 484 XP if dividing by 5 rather than 4, and looking at the replay, that's exactly what the xp number shows: 484.
Current XP
10811
16407