Why the varation in billing/usage periods

[#1]

Up until the last period my billing/usage period has been from the 22nd to the 21st of each month, but according to the VMBU the last period was 22nd June to 25th July and the current period is 25th Juy to the 21st August. (Screenshot attached) I always thought the useage limits (60GB) was based on a calender month. Is this just an error in the VMBU system? or is the usage period variable.

[#2]

This was due to an internal problem with usage resets on the 22nd July, these weren't reset until 3 days later hence the difference in usage period.

[#3]

Thanks for the explanation Chris.