Why local Isha prayer times may differ by roughly ten minutes

Answer

If the shaykhs involved do not elaborate on the reasoning behind their conclusion, no further clarification is possible from this end. Possible reasons for the difference include: (1) differing interpretations of shafaq (twilight) -- whether it refers only to red twilight (earlier time) or includes white twilight (later time); (2) whether the determination relies on direct observation, the standard position of all Sunni schools, or on astronomical calculation, which most people rely on today. Where astronomical calculation is used, there is debate over the correct latitudinal degree: some councils (Saudi Arabia, Karachi) use 18 degrees, others (much of the US and Europe) use 15 or even 12 degrees. The 18-degree method is the strictest and produces the latest Isha time. So long as observation of the night sky does not conflict with the CIOG calendar's timing, there is no reason to object -- if one does object, the burden of proof rests on the one raising the objection.