Sirius didn't know the likelihood of someone gaining control of another person's journal. Stealing a hair, somehow, and drinking polyjuice didn't seem the most insane though. There were ways of impersonating another person. Frankly, Sirius didn't know Frank well enough to have much to go off of for that. With James and the other marauders, it was as easy as breathing.
"It's not how I'd bet, but I agree," Sirius replied, knowing he had stated how he would bet on their mothers the night before. Whether it was Frank or not, the person either had controlled or read the journal. And that increased the odds. Somehow even talking about what they wanted to eat made him think of Remus, perhaps because of the full moon or perhaps because Sirius was a kinky son of a bitch. Either way, it certainly made him grin. "I think I'll go for the roast beef," Sirius replied.