First experience at buying expired domains

I want to buy 2 expired domains. I've checked the keywords they used to rank for, and some are still ranking according to Spyfu or SEMRush. But, when I check the "still ranking keywords" in Google, sometimes they appear in the results and at other times they don't, and if they do they don't have the same rankings as shown in both SEO tools. Any clue why this is happening? Thanks.