Phonics, this is how accents develop. Language has the influence of the environment and leads to the variation in the way we pronounce things. The primary language influences any other that you may learn.
For instance, South KPK replaces 'R' with 'Wa', such as someone from Northern KPK would say 'Ob Rasha', 'Ob' = water, 'Rasha' = bring; however, in South KP, you'd hear, 'Ob Washa', 'Ob' = water, 'Washa' = bring.
When someone from North KPK who's primarily spoken Pashto begins speaking Urdu then this comes back again as
You'd hear them put stress on 'R' in words - 'Rasta', 'Rishta' etc., However, Southern KP, new Urdu users would often unknowingly soften their 'Rs' with a 'Wa' south which ends up being 'O-W-R' so their 'R' words would stand out and they would be visibly focusing on speaking them properly.
There is a scholar in KP who's researching why this is so and they told me that this is most likely because Northern KP had a larger influx of Persian speakers which stresses a flow of the language. However, southern KPK had a greater influence of Dari and were largely ignored therefore developed without that influence. Hence, you'd see this difference. I'm sure when that scholar finishes their research they'd be willing to share their findings.
What you're asking has the same mechanics involved.
