I'm dealing with Springer about a monograph proposal. My draft was 80,000 words and they asked me to make it 85–100,000 words. I don't want to bother the editor with a question that is just out of curiosity. But, why do they need to expand it? What difference are 5,000 words going to make?
This is probably an economic decision based on the balancing of input effort/cost and likely return on investment. But you should ask the editor to see whether it is a "hard" rule or not and if you ask that, then you might as well ask why. I don't know if they make exceptions.
An exception might be "above the pay grade" of an individual editor, or not.
Here's an educated guess based on my experience with other publishers. They ran an analysis to see which of their monographs are most profitable (or receive the most citations, or whatever other metric they care about). It turns out that there is a correlation between length and profit. Therefore they ask for a certain number of words.
It's almost surely not a hard requirement, they'll probably accept an 80k-word monograph if you adamantly refuse to lengthen the manuscript.