About six months ago I entered the UK the second time on my visitor visa which I've already used once. In my landing interview, the border staff asked how many days I've stayed in the UK for my last visit. I answered X days (but in fact I stayed less than X days), and only realised that I answered it wrong after the landing interview. It was because the last visit was so long ago that I didn't remember that clearly. But during the interview, the staff didn't say anything; everything was quite smooth. I worry if this would affect my future visa application since in theory I should have given accurate information?
No, this will not in any way hinder any future visa applications, understaying for any reason is not frowned upon by any country immigration. Overstaying however, even though not overstaying your visa permutations can sometimes bring some issues, I met that first hand in Vietnam a few years ago where I stated I was staying X days but instead ended up staying Y days which was till within my visa allowance but on exit the immigrations lectured me about it