How are UTXOs handled in Ledger Live?
Silly me. I figured it out right after I posted the question.

Ledger Live shows the sum of the following:


Actual spent amount
The transaction fee


In my case, this is:


0.3878 BTC
0.00031680 BTC


Which is indeed equal to 0.3881168 BTC. I first got confused by this value because I subtracted 0.00065121 BTC (my newly created  UTXO) from 0.38876801 BTC (total input), but this is just a natural mathematical equivalence.