Why do my resources go down for no reason?

I've been playing AoM for a very long time, but I've never really noticed this before. I was playing a few random map games and I saw that my resources (particularly wood) were slowly trickling down for no apparent reason. Is there something I'm obviously missing? It only happens in late game when I have quite a bit of resources so it's not exactly detrimental just more confusing. I noticed it a Kronos run and a Poseidon run, just wondering if there's some mechanic I'm not familiar with.