Why prayer times differ between cities
Pakistan has one clock but many sunrises. A city further west sees the sun later, so its prayer times are later.
Longitude moves the sun
The sun rises in the east first. Pakistan runs on a single time zone, so a city further west sees sunrise and sunset later on the same clock. Karachi and Lahore can be around 20 minutes apart for Maghrib.
Latitude changes the length of the day
Cities further north have longer summer days and shorter winter days. That pushes Fajr earlier and Isha later in summer. The effect changes through the year, so a fixed offset between two cities never holds for long.
Use your own city
A timetable from another city can be wrong by half an hour. That matters most for Sehri and Iftar. calendar.pk has a page for every major Pakistani city, so use the one you are in.
Common questions
Why is Sehri in Lahore different from Islamabad?
The two cities sit at different longitudes and latitudes, so dawn arrives at a different clock time in each.
How big is the difference between cities?
Usually a few minutes between nearby cities, and up to about half an hour between the east and the west of the country.
Can I use another city's timetable?
Only if it is very close to you. For Sehri and Iftar, use your own city.