Since it's purely an Indian mindset to view everything through a colored prism of caste and ethnicity, then please explain the Pakistani mindset which results in Punjabi dominated Pakistani Army and negligible representation of Sindhis/Mohajirs in PA.
The Pakistan Army is not Punjabi dominated. I am a "Mohajir", & my grandfather was a Lt-General (rtrd), one of my uncles is a Lt-General (serving), & my cousin's husband is a Major-General in the Army. There are countless Brigadiers in my family as well.
Pakistan is also a Punjabi dominated country, as Punjabis comprise of about 55% (this includes the Saraiki, South Punjab people as well) of the country's population. And for your information, the Pakistan Army is a volunteer Army, so only those who apply can be enlisted.
A rough estimate shows that Punjabis comprise of 55.8% of the Pakistan's Army, Pakhtuns 15.4%, Baloch 2.4%, Sindhis 15.1%, & the remaining from Azad Kashmir/Gilgit-Baltistan/Urdu speaking communities. This is pretty comparable to the Pakistan's population by ethnic group.