RRD graph step don't match with script step

Hello.

My rrd file is fed  every 5 minutes, at 00:03, 00:08, 00:13, 00:18, etc.
But my graph looks like as if the step was at 00:00, 00:05, 00:10, ...
And so when I make a fetch on the rrd file. (I checked the unix timestamp).

Does anyone ever see this kind of problem?

Thanks a lot.

enerjaizer
P.S.: sorry for this poor English, I hope it's at least understandable.