What level of statistics logging set for dat, week, month and year, if there is a different setting then the parameter displayed for week, month and year will change. set week and month for the same level of logging as for day and also make sure you follow best practise in setting statistics log level with reference to KB http://kb.vmware.com/kb/2003885
Refer link and below: (vSphere Documentation Center)
To change a collection interval attribute, select its row in the Statistics Interval section and click Edit to open the Edit Collection Interval dialog box.
a In Keep Samples for, select an archive length.
This option is configurable only for the Day and Year intervals.
b In Statistics Interval, select an interval duration.
This option is configurable only for the Day interval.
c In Statistics Level select a new level interval level.
Level 4 uses the highest number of statistics counters. Use it only for debugging purposes.
The statistics level must be less than or equal to the statistics level set for the preceeding statistics interval. This is a vCenter Server dependency