manufacturing variances... just twist up some poly-fill and stick it in behind the filter card.
Water hardness can have this effect as well. The head on the bottom of the whisper air powered filters is supposed to produce really fine bubbles but often doesn't. If you go to wally and get an air pressure manifold:
You can regulate the pressure going into the filters. This one is set up duplex to run two of those tom's foam bio-filters behind it.
The left hose comes from a Hawkeye pump, first valve bleed goes to the filter behind it, second bleed is pressure venting and the right hose goes to the other filter just visible through the first tank.
For use on just one tank, gently close both valves, they're sensitive, put the air into it on either side and the out-to-filter on the other side then simply adjust one of the knobs to let enough pressure off to run the filter at the rate you want.
There are also back-pressure valves but I prefer to use venting as it doesn't wear out the air pumps or make them hotter.
Another valve type is kinda neat: If you take a T connector and run about a foot of hose off the T side then put a wooden cooking skewer up inside the open line, the back-pressure from turbulence around the wood can be used to adjust the output pressure from the pump to the airhead/airstone. This relies on air flow to be wasted but maintains a fair ratio of pressure when the water level is increased. Statistically it's cheaper since you can get skewers for a buck and a T pipe for a buck. Pipe organs often use this kind of tuning system.