LastOutput represents the last line of output generated by the command block. Since that line is a string, the /execute store command returns the length of that line (which is 84 in your case).
Instead of reading the number from the output, you can access that number directly:
execute store result score <player> <objective> run fill <x> <y> <z> <x2> <y2> <z2> <block>
this will store the amount of blocks affected by the /fill command into the specified objective.