Yeah I've seen this happen before. Somehow the storage count is getting out of sync and thinks there's more things than there really is. I've not been able to find the cause yet so any information would be useful. I suspect it happens when one bot is taking something out while another is putting something in at the same time.
I also think it might be when 2 bots try to put an item in a storage when it's almost full, so because it might be programmed a particular way the count says 801 but the code says 800, because it can't go over 800, making it out of sync. I have absolutely no proof of this though, but for some reason I thought it might be the cause