The estimate was calculated from the last change in your application, then counting down.
Like, it estimates to have next step within 2-4 months (depends on which step you are at, or which milestone I think) from the last change made to your application (change of address, you uploaded something, etc.).
I uploaded one or two files, my estimation back in August reset back from 20 days to 4 months.
I changed the address last week, the estimation reset from 0 days to 2 months.