In the rules on "Combining Magical Effects" (PHB, p. 205), the bless spell is given as the example of spell effects not being combined (not 'stacking'):
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.
I'm with this all the way up to the final semi-colon.
I get that the target cannot benefit from two of the same spell at the same time; I don't understand why the target doesn't get to roll two bonus dice.
The description itself says "the most potent effect—such as the highest bonus—from those castings applies". In the specific case of bless, the bonus is variable and is determined in each instance referenced by rolling a d4. It is impossible to know which of the two effects is the more potent without rolling two dice.
The process described of taking the higher bonus but not combining the effects seems to me to dictate that a target under the effects of two bless spells would roll two bonus dice, would determine which one was higher, and then would be affected only by the higher one (not combine them). Such a procedure would fit the requirements that the spells "don't combine", that the target gains the spell's benefit only once, and that the most potent effect applies. To not roll two bonus dice would mean that the target was not necessarily receiving the more potent effect.
To me it seems that rolling dice is not gaining a benefit from the spell, using the bonus rolled is gaining a benefit, and that is done only once.
What am I misunderstanding?