As @MadTuna suggests it could be decay is a factor with more data imported but your signature values will settle in regardless as more BT events occur.
TP isn’t necessarily the same as FTP although many users report similar numbers.
A traditional FTP test doesn’t measure the same values as what Xert is looking to determine your fitness signature
There are lots of newbie tips posted in this thread if you haven’t read it already –
Beginner questions - Support - Xert Community Forum (xertonline.com)
If you do decide to add the historical data back you can likely figure out what caused the difference by reviewing the data under Activities, Table. Or file a support request with support@xertonline.com and they’ll review your data and make an adjustment if necessary.