Question on 728 X 90 ad layout

1 or 2 ads in this ad layout

[conjo_guam]

Sometimes i see, 1 or two ads in 728 X 90 ad layout.
Does this mean, they are paying by impressions?

Normally there are 4 ads in that box.

My website is not popular and receives under 1000 ad impressions per day. Don't know, why and how anybody can target my website for 'pay by impression'.
Any help is greatly appreciated.
thanks

[sharegm]

I think this is normal, it doesn't mean paying by impressions.