Finishing any sizable work always takes more time than expected, more than estimated, more than it should. Why?
Because for every work that you do, you have external dependencies. There are small bits of things that need to be done by others for you to complete your work. And those small bits of work are as important for the whole work as the big chunks are. Just as a small needle in a sewing machine is as important as the big motor driving the machine.
And those small bits of work need to be done by others. And even though these small things hold the highest priority for you, for others, it may be just a side thing; an errand to be run once in a while or when there is a mood.
And surprisingly, there are just too many of these small bits of work in any sizable amount of work. And unsurprisingly, because of their size (or rather lack of it), these bits are never visible when you make your grand plan. When you make the grand plan, you see big things and budget for them. But small things? Because they are invisible, they won’t be budgeted for. And they’ll take longer to finish because of external dependencies! After all, your highest priority item is just another errand for others 
Caught ya!
Anyway, there is no point in holding a magnifying glass and looking for small bits when estimating time. It’s too much of effort and you won’t be able to do it anyways (remember, these are too small to be seen when your mind is occupied with big items).
Just go by the rule of thumb - it takes twice the time, twice the money and will give you half the returns than what you expect.