The maths behind it
We take the average interactions per post and divide by followers. Say 10 posts pulled in 4,100 likes and 220 comments. That's 432 interactions a post. On 12,500 followers, it comes out at about 3.5%.
Why size changes everything
A 600-follower account is mostly friends, family and regulars, so a big slice of them react to everything. At 60,000 followers, most people only see some of your posts and engage with fewer still. The rate naturally slides as you grow, which is why the result is compared against accounts in your own band.
Nudging it up
Post when your audience is awake, end captions with something people actually want to answer, and reply fast to the first comments. All of this tells Instagram the post is worth showing around. A free set of likes straight after posting helps that first hour along.