We're all fluent with numbers. We all learn to count:

• By ones (1, 2, 3, 4, 5, 6, 7, 8, 9)
• Then we learn to count by twos (2, 4, 6, 8, 10, 12, 14)
• Then by fives, (5, 10, 15, 20, 25)
• Then by 10s to 100. 

It's very easy, right? We never have to think about what number comes after 7, you just know these things. This comes from practice--from learning and rehearsal and the time that was afforded us in school to be able to do that. The problem for multilingual learns is that while you were practicing your rehearsal, and practicing that fluency, and getting better at it (over learning it), multilingual learners were just learning it. So they didn't get that same time as they moved up in grades to go back and get that type of rehearsal. 

A good example would be, to ask you to "Count backwards from 100 by 7." It's not so easy is it? That's because without that practice, there's no automaticity and retrieval fluency. For multilingual learners, it's not because they lack ability but simply because they haven't been given the proper instruction at the proper time and then the opportunity to be able to develop that same automaticity and retrieval fluency.