Looks to me as the LG has a problem to find a good write strategy above 12x and after some tries it decides to go for 12x on the rest of the burn.
For the Ritek burn, either your burner only supports those at max 8x or you selected 8x as write speed.
Pioneers are known to "jump" during the burn, so that graph looks like a normal one to me.Here is an example of a 16x write on one of my Pioneers.
Only a quality scan test can show which one that burned best. The posted images only shows how the disc was written. Not how good the burns were.