I'm currently doing some double-spend testing, and it appears that the standard client prefers broadcasting transactions that credit its wallet. That is to say, a client that receives such a transaction broadcasts it more, than it would broadcast other transactions in hopes of getting it into a block sooner. Is this truly the case, or are my observations incorrect?
According to the Satoshi client transaction exchange behavior, this does not seem to be the case. However, that page isn't written well. Looking at the actual source code, I cannot see any special behavior for transactions to an address in your wallet, but I do see special behavior for transactions from such an address.
Thus, I do not think the behavior you observed is actually valid, and is more likely a pattern you noticed because human beings are too good at noticing patterns for our own good.