The usual explanations, in order of likelihood.
THE RATE CHANGED
Point allocation is set by ZillionKingHost and can be increased or decreased as the company issues updates. If you calculated an expected balance using a rate from months ago, that is often the whole answer. Check the current rate on your Rewards dashboard.
AN ASSUMPTION ABOUT WHAT QUALIFIES
Domain purchases, promotional orders, add-ons or renewals may not earn at the rate you assumed. The dashboard shows what currently qualifies.
A REVERSAL
An order cancelled or refunded removes the rewards it generated. Check the history for reversal entries.
EXPIRY
Where expiry currently applies, an older balance may have lapsed.
REWARDS ON AMOUNTS SETTLED WITH CREDIT
Amounts paid using rewards or credit typically do not themselves earn. Rewards do not compound.
A PENDING BALANCE NOT YET CLEARED
Check whether the figure you are looking at is available or total.
WHAT TO DO
Read the history first, then compare against the current dashboard figures. If neither explains it, open a ticket with dates and figures.