One of the things you must realise is written swahili is not so popular such that non-natives dont know the classes as they mostly learn through speaking Ive not been taught these classes in school and they mostly come from practice and listening. Now imagine if the speaker himself isnt using the correct class. For example ive seen many use yangu in place of the wangu or the correct replacement Many use ma- for plurals even if it isnt like magoli, makompyuta etc. As a sort of emphasis I previously mentioned badilisha being used almost always instead of badili You will hear a lot of swanglish in dsm. A lot.
In dsm this is the situation however someone from zanzibar(not mainland) would know better swahili but lesser english. They are more traditional there.
So you cant really expect to have someone perfect in both. In dar due to the population greatly being indian, instead of spices they use magasia Ma- (for plural) gasia means something unorderly and extra like piles of old clothes. So here it means extra stuff for the food