Recipes with part LME and part DME

Sometimes I see recipes that call for both LME and DME to be used. Usually, it's LME for the bulk of the extract, and then a smaller amount of DME. Why would you do this? The only thing that I can think of is maybe it has to do with the size of the containers that you commonly find the extracts in - for example, if you needed 7 lbs LME, it would be a shame to have to by a 6 lb container and a 3 lb container and toss the other two pounds. So maybe you would want to use a 6lb container of LME and then (part of) a 1lb bag of DME to finish it off. It wouldn't really make a difference if you used all LME or all DME, would it? I can pour LME into my own containers at my local HBS and pay by the pound, so it would be no problem for me to get the 7lbs of LME.
It's not a big deal, I'm just curious why a recipe would call for both.

i think you hit it on the head with the sizes of the LME containers

I don't know the reason recipes call for one over the other (or both), but I think you need to make an adjustment when you substitute.

If you sub LME for DME you should add 20%, and if you sub DME for LME you should reduce 17%.

If you don't adjust it won't hurt your beer, but it might change it a little.

1 lb grain = .75 lb LME = .6 lb DME

In many HBS LME is available in 3.3# cans. These are not resealable and LME doesn't store that well any ways. In some cases it is available in 3# or 6# jugs. DME is usually available in increments of 1#. DME is easy to measure and store but usually more expensive. So if I was making an extract batch I might use 1 or 2 cans of LME and then enough DME to bring my gravity to the desired level.
Since you can buy LME by the # just replace a # of DME in recipes with 1.2# LME. Should get very similar results.
Craig

Thanks for the answers! It looks like it's more a matter of convenience than affecting the beer in any way.