"after rolling out our optimizations, we saw our error rates (ie. the proportion of slow or timed out queries) drop by up to 400%"

There is some good shared knowledge in the post (unlike this comment, to be fair), but what does drop by 400% mean?

If a rate drops by 100% it becomes zero. I get that.

If it increases by 400%, the outcome is slightly ambiguous (do we add 400% for 500% total or do we multiply up to 400% of the original value).

But a rate decreasing by 400% - am I the only person who finds that (not uncommon) expression hard to conceptualize?