Seen this many time too..
I've attributed it to the OP changing their minds, if they choose a new solution the system does not remove the notification, which kinds of makes sense..
regards.. ~q
I believe it is not so much that the OP has changed his mind.
He just marks everything he thinks has contributed to the solution and isn't aware that the next solution mark removes the previous one.
Should be improved in the forum software. For example with a hint that only one solution is possible but one can use the heart/like button several times. Or at least, that the PO has to remove the first solution before he can set another one.