Don't let it bug you. If you include all the Roman numerals, you can get almost any answer you want (and no doubt they'll reject yours and accept someone else's).
ELEVEN includes L and V, SIX includes I, X and IX, PLUS includes L, FIVE includes I, V and IV. They may consider IM means 999 (which it doesn't, that would be CMXCIX), so that TIMES includes I, M and IM . If you add up all those, and then add 11, 6 and 5, you get 2157. So you can get 2228 by including the 71 which is the original "obvious" answer.
I've surprised myself there, I didn't expect to be able to get to their answer. But I'm not going to waste my money ringing them up now I seem to have cracked the code. You could justify taking 121 instead of 71. You could omit the 999. So you could have rung up with 2228 and they'd still have said incorrect. Don't get sucked in!