You'll find with sites like alexa.com and similiarweb.com that the bigger the site, the more accurate their estimates are.
Basically, if your ranked above say 200k on alexa then it only takes a small amount of traffic to make a big difference to your ranking.
However, under 200k and it takes a lot more traffic to change your rankings. The closer you get to rank 1, the more traffic you need and the more accurate your ranking will be
Its to do with the
Long Tail Effect
This is why some people think sites like Alexa are rubbish and to be honest, they are not much good for smaller sites but very useful for larger sites
hope that helps