I’d imagine the spermicide exists to ensure the lubricant on the condom doesnt act as a transport for sperm, not as a primary method of efficacy.

Looking at it as a mechanical problem: Sometimes in condom usage, I’ve noticed that a little air that gets sucked into the condom, and then works its way out again. So clearly theres some limited fluid exchange possible at the base of the shaft, which may then end up as sperm in places its not intended to be as the use of the condom continues.