There can be alot of reasons why a player suddenly spikes in total strength. The player could be:
- Hoarding alot of Manyus and use them all at once on many girls.
- Giving girls alot of affection (with gifts).
- Getting better and, (or upgrading) equipment.
- Learning how to level up effectively (for example not to use a wrong color Manyu on a girl).

Edit: Reading your post again made me a bit confused. I'm assuming you are talking about strength, the thing you see on the Ally Power Ranking. But you also talk about experince and leveling.