Why does connecting an account to online service for the first times, affects the balance?

I have a Health Equity HSA Savings Account. I never connected this via online and manually added transactions. I decide to connect it to the online service. When I did, and it connected to the first time, I found out that my ending balance for that account was off. For example:

Before I connected to the online account, My Balance was: 4886.11
After I connected to the online account, My Balance was changed to 2288.37 and it downloaded 128 Transactions.

NOTE: I didn't accept any transactions or anything. Do you know why this happens? It basically corrupted by data and now I have to restore from a back up?