I don't know much about VC, but it seems like this can't possibly be true. Aren't there times when a VC invests, and then declines a followup round because they

I don't know much about VC, but it seems like this can't possibly be true. Aren't there times when a VC invests, and then declines a followup round because they still like the company but don't want any more exposure to their industry or country? And if a company is doing a follow up round valued at $X, any estimated valuation between $0 and $X-1 should be low enough that the VC doesn't invest in that round. There's no reason to assume it's always $0.