I have never been in a project, where estimates were spot-on, and I do this for 15 years now. By now hundreds of features have floated by the river and hundreds of meetings have been held.

Estimations are often complicated, because there are way too many variables involved to give accurate estimates. Politics within companies, restructuring of teams. The customer changes their mind, the reality you've expected is slightly different, your architecture has shortcomings you find out late in a project, the teams your work depends on disband, … and a million other things.

Theoretically you could update your estimates in a SCRUM meeting, sure, but to be honest, this has always been nothing but a fantasy. We rarely do work in a void. Our features have been communicated higher up and have already been pitched to customers. In a fully transparent and open organization you might update your estimates, and try to explain this to your customers. In reality though? I have never seen this.

While this sounds very negative, my take on it is not to waste too much time on estimates. Give a range of time you expect your features to fall into, and go on with getting your work done.