Excel is a great employee 5 days a week, but on weekends, he release his stress and becomes XL!
Excel has 5 letters and XL has 2 letters. 5 days at work, 2 days running against the Speedrunners.

With that said, you can play with Excel or XL depending on the day.

*Note: I'm not sure if he becomes XL on holidays. Probably he does.
*Thanks to `Shao Long and kickman199811011 for the additional information.