In practice, if it's adult genre fiction under 75 kwords it's going to be treated as a novella by print publishers for purposes of determining whether it's publishable, so... Well, that's my excuse and I'm sticking to it. But this is why I get nervous about writing from synopsis to specified length on a contracted book, which is what I had to do for the novella in the New Year's Eve anthology last year. That one had to be 20-35 kwords, and when I started I thought I was going to be under the minimum, and by the time I hit 30 I wasn't convinced I was going to be able to keep it under the maximum.