Assalamo Alaykom. Everything is in the title, why is Dhuhr prayer time fixed in Algeria? I'm using a software that indicates prayer times, all other prayers are correct except Dhuhr. I tried all calculating methods but none of them indicated 13:20 for Dhuhr. If you have clarifications I'd be thankful. Salam.
I figured out why.
As season changes, the length of days increase in summer and decrease in winter. However, the midday is always 12:00. So the shrinking/growing of day time is always symmetric around 12:00.
Now the zuhr time is set when sun is at a certain angle, but very close to midday. When season changes, the two ends of the daylight changes drastically (fajr and isha), but the zuhr prayer is very insensitive to change, because its change of time is only a small fraction of the change of time in fajr. The exact fraction is roughly estimated 1/9. (10 degree /90 degree).
So during a week, the time of fajr usually don't change more than 30 minutes, that means the time of zuhr changes no more than 2-3 minutes. So you might not observe the change.
This is my hypothesis, or maybe it is set to be constant, because it is easier. 12:00 is always midday, and 13:20 is when sun is about 10-15 degree after midday, which is fine. Both are possible.
In conclusion, it doesn't matter too much because of the scaling factor.