What sequence memory measures
This is a working-memory test: holding an ordered list of positions in mind for a few seconds and reproducing it. Classic span research put the typical human limit at "seven, plus or minus two" items; on a spatial grid like this one, most people top out somewhere between level 7 and level 10.
What fails at your limit is interesting: usually not the tiles themselves but their order — you remember which tiles lit, and lose the sequence. That is the signature of working memory running out of rehearsal room, not attention lapsing.
Getting further
The reliable technique is chunking: instead of nine separate positions, hold three little three-step paths — corner-middle-edge reads as one shape, not three tiles. People who reach the high teens are almost always chunking spatial patterns, not brute-forcing longer lists.
Rhythm helps too. The flashes arrive on a steady beat, and replaying that beat in your head while you tap preserves order information that pure visual memory drops. Like every memory task, scores climb noticeably over your first dozen runs — that is real skill, not just familiarity.