if your using the google bid tool parked uses a yahoo feed so there would be a difference as parked doesn't use a google feed and that tool gives you what people pay for the search network when someone actually types their query into the google search box, parked domains pull their ads from the content network and some advertisers on google and yahoo exclude their ads as they don't wanna be shown on parked domains or they lower their bids for the content network which shows on parked domains, create a google adwords account and you will see you can exclude your ads or use lower pricing for that network, move it around try a few google feeds sedo, namedrive, domain sponsor, fabulous and try a few yahoo feeds parked, trafficZ, smartname or whoever will give you an account and stick it where it does best, I have a few favorite parking companies but I wouldn't declare anyone the best for all categories of domains regardless of what anyone says as I have used all the companies I mentioned plus a few others as well, and as murcc2 said smart pricing or quality pricing is another factor if your clicks never convert for the advertiser your payouts will be lowered which counters people clicking their own ads or having their friends do it as if it never converts your whole account will be looking like .01 clicks.
