Indeed the word "spam" itself is perceived by the spam filter as being spam-like
LMFAO WUT
Unfortunately I can't tell you why, because anything that helps spammers evade spam filters is bad
Do spammers even bother to look up how to evade a specific site's filter, though? I'd have presumed most of them are partial-automated or being done by people in third-world countries that don't speak English fluently, where the alternative is shovelling **** off the streets for a penny a week.
Often poking around server logs you'll find tons of attempts that never go through, such as trying to login with credentials that don't actually exist or attempting to post new threads in whatever forum ID 1 (or the first forum in the index listing) is.
Sites that transcend across Internet culture like 4chan and Twitter tend to lend themselves to specifically-tailored scripts and people who explicitly target those platforms, but most of the time forum/blog spam is done the same sort of way and rarely do they actually "adapt" that well, since a simple catch-out like that throws the majority of them off and the few that are left are so minuscule (a sortable Q&A that pertains to the forum's topic, with a question like "Which cars are German and which are Japanese?" would be time-consuming for spammers to Google the answers to, in which they'd probably just go find a more vulnerable place to post their crap that's quicker to sign up on.