Lots of factors can affect that, one being boil time and the boil off. While the sugars in the kettle will remain the same, the liquid content will concentrate the ratio of sugars to liquid. Just one example.
I have my efficiency set to 68% for my mash but on the latest attempt on my own W12 clone recipe I recorded 70.6% mash efficiency. Once fermentation completed, my final numbers reflect a total efficiency of 78.7% from start to finish. I did a 90 minute mash with a 1.5qt/lb mash ratio. I collected 28L of wort at 1.062 and ended up with 22L of wort an OG of 1.088 after a 90 min boil. Fermentation took around 3 weeks to finish up before I could start crashing it.
79% total efficiency isn't too bad and even I get confused with how the numbers are calculated. That's why I let Beersmith do all of that math stuff. I'd hate to see how my beers turned out if I relied on my algebra skills.
Mash efficiency is just a measure of [actual fermentable sugars from your mash]*100/[fermentable sugars theoretically possible from that mash]
Beersmith [or whatever] will give you theoretical or potential fermentables for a given quantity of grain or adjuncts and then you will measure your SG of your wort before pitching and compare the potential SG with the actual SG.
Boil off just decreases the volume. You need to know the real volume so that you can generate your theoretical SG correctly from your Beersmith (or whatever.)