1979 Toyota Cressida Ignition Timing

Engine Mechanical problem
1979 Toyota Cressida 4 cyl Two Wheel Drive Manual
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My car is Cressida 1979 CX30 model having 18R engine of 1968cc 4 Cylenders. Point type Ignition system.
Ignition timing is 10BTDC on 600rpm.
Firing Order 1-3-4-2
My Question:
When I use Timing Gun to set ignition time I observed that Timing light patterns (flashing pattern/frequency) are different for piston 1&4 and piston 2&3. Mean flashing frequency for 1&4 is slower than 2&3. 1&4 are same and 2&3 are same.
What is the reason ?
Even point gape and ignition cords and firing plugs are ok.

Optional Information:
Pre- 1983 Cressida CX30 18R

Already Tried:
Just overhaul my engine and now with new fire plugs & ignition cables and ignition point.