Checking the seatmap for an upcoming flight on UA840 SYD-LAX and noticed that 37 EF were "reserved" while 37D and G were available. 36DEFG and 38DEFG were all available, so it doesn't appear to be a group split across rows.

Similarly on UA145 ORD-SFO, I see 14E reserved while 14 D and F are available. (Though in this case 13DEF and 15DEF are all taken, so it could be a split).

I can't imagine why anyone would want to reserve the middle seat only