Suppose in local time (which is, say, UTC+3) the DST transition is on something like "last Sunday of 10th month at 02:00", which might then in UTC become "last Saturday of 10th month at 23:00, except if that Saturday is the last day of the month, then the second to last Saturday". In effect, if conversion to UTC causes the transition to move to a different day, it might also be on a different week or month, causing headache.

(Or did you ask something else entirely? It wouldn't be the first time I misunderstood things about time zones.)