A 10-10 round is an extremely hard standard to meet by letter of the law. In order for it to be scored, you have to have the Effective Striking/Grappling be dead even. Then you have to go to the secondary scoring criteria of Effective of Aggression -- i.e. making active efforts to finish the fight at every opportunity -- and make the determination that both dudes did just as well as the other in that regard. Then, finally, you have to go to the tertiary scoring criteria of Fighting Area Control -- and ask yourself who was dictating the three P's of Pace, Place, and Position of the round across the five minutes? This last one means that even if you judge that both guys were equally effective in their striking/grappling and both were equally aggressive, the one who had a few more seconds marching forward and putting the other on the back foot behind the black lines or holding him in the clinch with zero strikes thrown can win a 10-9 on that basis alone.
A 10-10 round is supposed to be one in which all three of the scoring criteria listed above is dead even and in which it is effectively impossible to draw any distinction from one competitor or the other. I won't say it never happens or has never happened, but it is a damn difficult standard to meet in practice with the Rules-as-Written. This is exactly why I like the idea of half-point rounds like 10-9.5 and 10-8.5.