One problem here is that roll20's Keep mechanics don't work with exploding dice. Say you rolled 5 dice, and 3 of them explode adding 1 each. If you have a skill of 3, it would just take the best 3 of those 10 dice. You might expect it to keep the best 3 dice, and the extra 3 exploding dice that went with them, totalling 6. But it won't, it'll just take the three best, so you'll end up with three tens. Using the !! modifier for exploding dice causes any extra dice to be added to exploding dice, and it is treated as a single die. So, for example, if one of your dice rolled a 10, and then a 5, it would be shown as a single die of 15. This ensures the proper sum is used, but counting raises is trickier - you have to remember than any die showing above a 10 is actually 2 or more dice, with one or more 10s included.