Old Bitcon Core wallet shows X amount of BTC, but when checking it on bitcoin explorer it's less than half of X?

First of all: Yes I will not show my private keys to anyone and yes I will not respond to any DMs.

I found my old Bitcoin QT / Bitcoin Core wallet some time ago, theres some amount of BTC on it. I hate having to sync the whole change in the blockchain each time I run Bitcoin Core after a few months, so I'd like to move all my funds to a different kind of wallet. I heard about Electrum and Exodus.

All of the transactions (reveiving and sending) on the wallet have been made in 2014.

I can see 4 receiving adresses in the history of the wallet.

When I enter these 4 receiving adresses in a bitcoin explorer, only two of them show funds which in total accumulate to only half the amount of what Bitcoin Core is telling me.

Any idea what I am doing wrong here?