"I often swim alone."
Translation:Gyakran úszom egyedül.
This rule is not exactly set in stone. There are many exceptions and regional variations. Some verbs are acceptable in both versions, others sound really weird using one or the other. There are also "fake" ones that the rule does not apply to at all. And there are ones where the two versions may be used in different contexts.
So, they may have thought it good to accept both versions here.