If you look on the bloom package (10-30-20)at the ingredents you'll see Jacks has no Urea N in it whereas MG (15-30-15)has it in abundance. I think this type of N causes most of the nute burn. Urea takes a while to break down and stays in the soil and is hard to flush out. They do put 5% ammoniacal N in it though, which is much easier/quicker to be used up, and nitrate N which the plants readily use. It makes sense to control the amount of N being used at this stage.
Jacks all purpose has alot of Urea in it. (17.9%), but this is the time where N is crucial. So it makes sense to put a N in there that will be there a while. If you notice, they don't mix ammoniacal and Urea Ns together, I think its because they both are time release so theres danger of to much. MG mixes them all together, which is why I think it causes nute burn.
This is only imo, and if i'm wrong please correct me.