Measured OG vs Beersmith

Just finished brewing my first brew that I devised the recipe for myself. Recipe was as follows:

Boil volume - 6.25 gallon

Steeping:
1lbs crystal 60L
.5lbs cara-pils

Extract:
4 lbs light DME
3.3 lbs gold LME (late addition @ 20 min during boil)

Hops:
1oz cascade 60
.5 nelson sauvon 30
1oz cascade 5
.5 nelson sauvon 0

Yeast - US-05

I ended up with just over 5 gallons in the primary after racking out all of the gunk and pouring about a quart of gunk wort from the bottom of my kettle (after whirlpooling) out.

Anyway, my question is regarding my measured OG. Beersmith gives me an estimated OG of 1.060. When racking, I took a sample and using my hydrometer got a reading of about 1.052.

My understanding was that when using extract, the OG should be pretty simple to figure out - so what gives? What could I have done?

stratification. heavy gravity on the bottom, lighter at the top and you grabbed some of the lighter

as long as you hit your volumes, your gravity for extract should be exactly what your recipe says it should be. with an all-grain recipe, that 1.052 would tell you that you got a lower mash efficiency. that's not an issue with extract, because it's been mashed for you

relax, chill the wort, pitch the yeast and congratulations!

you've made beer!