correct preposition/s

Hi Yoong,
Longman Dictionary of Contemporary English says:
New Zeland was weak in defence.
She speaks fluently but she's weak on grammar.
Oxford advanced Learner's Dict. says:
I was always weak in the science subjects.

But I also checked on the net, and it seems to me that "strong" and "weak" are used the same way as "good". In other words, it seems to me that "strong in/at" and "weak in/at" are used very similarly to "good in/at", and "on" is not used very much.

Just my thoughts, I'm not sure. When prepositions come into play, things always get tricky for learners.