Hi
If you put that other modem back and resync again is it constantly down by the same amount or does it go back up? Perhaps you have just regained your position in the cross-talk rankings.
You also need to remember these are cheap consumer devices, they make many measurements to decide on sync-rate etc, yet aren't scientifically calibrated and no two are going to operate exactly the same. Rounding plus rounding errors in analogue to digital converters and software on the chips can result in the very small differences in tolerances of components ending up being noticeable in the end result and two seemingly identical modems behaving differently. Why did it change after being off, could be any number or reasons, temperature of components at the time of resync, difference in noise profile on the line and this modem due to different tolerances is slightly more conservative or noisy internally than another one. As it cooled down a decoupling ceramic capacitor could have cracked causing more noise or an electrolytic capacitor is close to degrading and the cooling then warming cycle due to being turned off pushed it further towards a fault condition. Even dust and the moisture levels on the PCB can change it's characteristics.
This other modem might have always been able to sync ~500K higher.
You also would need to do several tests with each modem to know for sure if one is consistently syncing higher, there is always an element of variably in sync speed on each resync. I've been there before, tried a different modem, surprised at how much better it was, set it all up properly to use permanently, and by the next day it had resync lower and ended up being worse, I put the original one back.
The short answer, no one can tell you why it is a bit slower with any absolute certainty.
Regards
Phil