My husband (later stage Alzheimer's) has forgotten almost everything, so how does he still remember some sexual things?

A lot of you already know my background, but here's a quick summary for those who don't: Bill, my husband, has later stage Alzheimer's and I'm his full-time caregiver. Bill is 73 and retired, but only has his modest social security, so I still work full-time to support us. I'm 64 and have a high-stress job, but I'm blessed to be able to work from home on Tues and Thurs - and then I take Bill to an Alzheimer's day care on my way to the office on Mon-Wed-Fri and pick him up on my way home. He's incontinent, speaks very little, understands very little and needs help with just about everything now, but is still somewhat ambulatory.

So here's my question: He's forgotten almost everything and doesn't remember anything from one moment to the next, so how does he still remember some sexual things? He gropes me occasionally and will even try to masturbate on occasion when he's by himself (unsuccessfully, however, since he can't get an erection). I have a baby monitor in my home office so I can keep an eye on him when I’m working from home. I don't resist when he wants to touch me if it makes him happy. I just can't figure out why this one thing is still present in his brain. I know it's crazy, but it makes me wonder if he knows or understands more than it appears.