Dispenser arrow do less damage than Player arrow when launched with tnt.

Recently my friend told me to design a tnt arrow cannon for his server.
There were many bugs I encounter while making this but I won't include them in this report.
The machine is suppose to be completely automatic, so it uses a dispenser to load arrows in the chamber.
But for some reason the arrows from dispenser do very less damage.
In the video I uploaded the damage is 9.5 hearts however that is not consistent either. At times it would 1.5-2 hearts of damage, the regular damage from a dispenser when the arrow is shot normally.
Since bedrock edition doesn't have advance commands like java it was very difficult to find the launch velocity of the arrow. But upon further testing and calculations it was found that arrow would travel approximately 87.5 blocks per tick or about 1750 blocks per second.
It's very much possible that my calculations and method. Its uses 180 tnt to launch the arrow.

Sorry for the poor video quality, I don't have a OneDrive account and I don't want my id on other sharing site.
Is case the video is to blurry, on the first half I tested with dispenser arrow without any armor and it did 9.5 hearts of damage and on the second half I tested with player shot arrow and was using full protection 4 diamond armor of it one shot killed me.