I FC'd a song. Why is my combo lower than people on the high scores?

Here's a video of me playing a beatmap. As you'll be able to see, I never miss a hitcircle or slider. Four times, I get 100 points instead of 300, but it doesn't kill my combo.

I end up with a max combo of 522. However, you can see on the left that the highest scoring players have achieved combos of 523 and 524. They have mods on, but nothing that should allow them to get a higher than normal combo. How is this possible?

My only guess is that I somehow had hitcircles or sliders that didn't add to my combo, but also didn't set it to 0. Is this accurate? If so, what triggers it? If I'm totally wrong, what else could it be?