The performance charts in vCenter vary in the collection periods and rollups. I think they breakdown as follows:
Real time = every 20 seconds
Daily = every 5 minutes
Weekly = every 30 minutes
Monthly = every 120 minutes
Yearly = every 24 hours
It sounds like you might be getting caught up in between rollups. It's best if you take look at the performance troubleshooting guide and see if that helps you out. It's a great doc and covers esxtop as well.