how accurate are home appraisals when refinancing?

I'm in the middle of refinancing my house and just got the appraisal back. Honestly, it came in lower than I expected - like noticeably lower. I've done some upgrades recently (new kitchen counters, flooring, etc.) and thought that would bump things up a bit more. Now I'm wondering how accurate these appraisals usually are...do they tend to undervalue homes or am I just being overly optimistic about my place? Curious if anyone else has had similar experiences or knows how much wiggle room there usually is with these numbers.