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?

You actually didn't full combo the map.

You failed to clear some sliders, which doesn't break your combo but doesn't add an extra point to it:

A slider is composed of a starting point, counting as +1 in your combo, sometimes middle points, for long sliders, each counting as +1 in your combo, and the ending point, also adding +1.

A long slider can be represented as such:

(start) (middle) (middle) ... (middle) (end).

In your case, most of the sliders are (start) (end), they are not long enough to imply some middle points.

Failing to hit a (middle) point of the slider results in resetting your combo, it is counted as a failed note.

But the trick here is that failing to hit the (end) point doesn't reset your combo, but it doesn't add the expected +1 and gives a +100 instead of +300.

You can see this at 0.49 and 1.27 in your video, you start your slider, move, then you release your key (or mouse button) before completing the end of the slider. Resulting in a +100 and not increasing your combo: you haven't fully completed the slider.

You did it twice, resulting in having a combo of 522 instead of 524.