New to Blockers

I am having a hard time understanding this part intuitively:

"If we intentionally leave out clubs in our betting range, Villain can stop mixing and start pure-raising flush draws to exploit us for betting flush draws-unblocking air at too high of a frequency."

Our bluffing frequency is still in check, so how does the fact that we are not bluffing with any clubs allow Villain to pure raise his flush draws?