Just an issue of dialects, therefor they understant both dialects, like Arabic dialects.

yes there is a difference,and in fact there is a difference in Pashtu spoken in Peshawar and other areas in Pakistan,like the one in Quetta(Baluchistan) too.. The natives can easily point out the difference in words and phrases they use

hmmm, za pohegum..dera manana!

w/salam... i think there is not much difference,the language is same,but in the pushtoo of pakistan some english and urdu words are uses,and in afghan the pushtoo is pure cose it is derived from there, but the people can understand both viseversa. means if u learn this language in pakistan u can understand and if u learn in afghan u can also understand...only pronoucation is different