That still could have been pure coincidence. Possible reasons:

1. Improvements in the blocking features of Gmail. As pointed out by others, some messages just get blocked at the gateway, never even getting the chance to hit the heuristic spam filter. It's possible that Gmail (or someone else whose systems are part of the relay chain) is making constant improvements to this, so the number of messages in your spam folder is naturally decreasing over time.

2. There are occasional news reports of certain spam-related resources being taken down. For instance, maybe a particular spam botnet gets ferreted out and eradicated. Maybe a particular open relay gets shut down. Maybe a particular company or individual who specializes in sending spam gets their financial channel cut off and they stop their activity. This kind of thing happens all the time, and when it does, we frequently see news reports of how there was a sudden drop in spam frequency on that day.

I would be very surprised to see that Gmail is watching what you delete and modifying their spam filters as a result. I think it even less likely that they are somehow creating a special heuristic spam filter specific to your account.

There's one exception to my statement, though, and that's this:

If, when you delete the message, there is some other option being made available, something to the effect of "always delete all messages from this sender", and you're selecting that option, then it would be automatically creating a filter for you, and might explain the behavior you saw. However, when I delete messages from my Gmail spam folder, I don't see an option like this available. So I'm doubtful that's what's happening in your case.