It has to do with the mechanics of the game, so it won't be this way for all sheets. The Pathfinder by Roll20 sheet is a good example of this behavior (that I am a familiar with). In the Pathfinder system, a roll that is in the critical range isn't an automatic critical. You need to roll another attack roll, and if it is high enough to succeed in hitting, then that second roll "confirms" the critical. Both of those rolls are made every time you make an attack from that sheet. The 3D dice show both rolls, because they are both being made within the dice roller. Chat doesn't show both dice unless the first roll is a critical threat, because the roll template is using a css rule to hide the confirm roll unless it is needed. Basically, the 3D dice are showing all rolls that are actually being made. Chat may not show them all due to the roll templates hiding some of the rolls. Not all systems have mechanics like that, so this won't happen with all sheets.