Different tools pull data from different sources.
I have access to KW revealler, LTP, Moz too, but I tend to try to stick to the same 3.
Ahrefs updates quicker than Semrush, I find.
Dan from KeySearch posted on here somewhere as to why different tools show differing results,
@dp40oz
I was, as an example, looking at a keyword Saturday which was scored 24 yet on Ahrefs it was KD46, on huge search numbers.
Later on I found a longtail which was 22 on Keysearch yet Ahrefs scored it KD0.
I think the more you use these, the more proficient you become at scanning your eye over the top 10, then if there's a hint it's viable, dig deeper using Ahrefs.