google paid me $5 less than finalized earning

I am a recent admob publisher and just started earning from past 2 -3 months

earlier payments were proper , this month i received payment which was $5 less than the finalized earning (after deducting any amount for invalid activity which was less than $1)

my bank doesnot charge any amount for conversions

has anybody else suffered the same fate either now or earlier

strangely even cpc has fallen with ctr for the ads

about a week ago i received a mail asking to join interstitial ads since i only use banners

even installs are down from about a 1000 a day to 700

is this normal or its just me who is suffering.

please share your experience.

Maybe its normal