Estimating measurements in an experiment leads to inaccurate results because they were guessed. Estimation is roughly guessing the results therefore it leads you to a wrong answer. By guessing you're assuming the answer, you don't know if the answer is correct and you don't know if that answer is the actual result. Say if you were estimating something in your science lab such as how much the battery has drained in a certain area, well using this you don't actually know if that's true and if someone were to use your results they may end up having a problem sense you ESTIMATED IT.
Sorry if this answer isn't sweet and simple but more explained, it's kind of hard explaining this. I hope this helped!