The other thing you can always refer back to is that the accuracy of an estimate increases the closer it is to finishing it - that is, it's very inaccurate at the start. This is one of the pillars of the agile way of working - you and your team can give a fairly accurate estimation for what can be finished within a week or two. Not so much for a year.

The longer estimations are very much due to changing priorities and a developing environment and understanding of what is being built though, hence the inaccuracy.