I don't really think the 2501 is just a derated 3200.It depends on what you mean by skirt. Taking a L3200 & debating it from 32hp to 25hp & calling it a L2501 to comply with tier 4 emissions is entirely legal. It was done entirely legally & with the entire purpose of coming in under the HP limits thar require all the extra emissions equipment. So if you mean come close to or work around a restriction by skirt, it does just that.
The 2501 and 3200 use completely different engines, the 2501 is a direct injected D1703, the 3200 used a indirect injected D1503.
A majority of the HP difference is that the 2501 has a rated engine speed of 2200rpm and the 3200 had a rated speed of 2800rpm
Its pretty common to change engine output via rpm and timing/fuel to meet certain applications, which is what Kubota did here.