Mainly because I've made no more than three small mirrors ranging from 5 to 7". All of them turned out fine.
Who made this mirror, a D = 249 mm F = 1290 mm, most likely had no time to spend in correcting the shape. Or he tried, unsuccessfully.
Each shot was taken moving the carriage (single blade moving source tester) away from the mirror by 1 mm.
Thx in advance