I believe they are based on US 15's mileage from the Virginia line. Exit 9 is a bit off (NB off-ramp is 10.3 miles from the line) but the rest line up and there is no Exit 11.
If the US 15 exits in Frederick are meant to be an extension of exit numbering it would make way more sense to have them continue I-270's exit numbering.
Exit 9 is at MP 10.3
Exit 10 is at MP 11.3
There *IS* an exit 11, it's the route 180 exit at MP 11.6 -- exit number only displayed NB but it's on the overhead BGSs and on the exit sign itself (saw them last week myself)
Exit 12 is actually at MP 12
but then exits 13 through 17 are crammed into a 2.5 mile stretch (MPs 12.6, 13.4, 13.9, 14.6, 15.1)
and exit 18 is at MP 16.5
That screams to me that MDOT wasn't and isn't using mileage to base the exit numbers.
I totally agree that it would make more sense to have exits 13 and up as extensions of I-270 exit numbers.