Does the online system get confused?

Last night my Oyster card balance was £2.65. I bought £5 top-up online to
collect at King's Cross St Pancras this morning and when I did the gate duly
showed my balance on entering as £7.65.

I went to Heathrow where the gate showed my balance on exit as £5.60 and
fare deducted (£2.05).

This was all as expected but going online just now my balance shows as £4.20.

Huh?

--
Colin Rosenstiel