BeeGee, our normal M.O. so far has been to default to whichever parent feels the strongest about something. Usually this ends up being the more restrictive parent, although to be honest we really don't disagree on much. In this case, though, MIL has a history of trampling on our parenting decisions and DH has a history of choosing to avoid conflict unless he feels very strongly about whatever it is. My guess is it went like this:
DH to MIL: Sure, whatever, cake is good!
Me to MIL: Dude, not okay! No backup cakes!
MIL to DH: Your wife is such a whiner! She said no! Can you believe it?
DH to me: What's up? Why do you disagree?
Me to DH: blah blah blah reasons
DH to me: She was just talking about a one-serving cake.
Me to DH: I guess that would be okay.
DH to MIL: She said a one-serving cake would be okay.
MIL to DH: Cool! Want to join us for lunch and cake?
DH to MIL: Sure.
*MIL buys the cake*
*DH sees this, realizes MIL didn't mean a one-serving cake at all, knows I didn't want her to, knows he doesn't care one way or the other, but sees it's making Babybartfast happy and doesn't want to confront his mother*
(Significantly shortened and with a healthy dose of snark and assumption, but . . .)