The Tiger came before the Panther, but the Tiger is Mark VI and the Panther is Mark V. Does anyone know why that is?
Because those designations were assumed by the allies. The VK 30.01 project was accepted as the Panzerkampfwagen Panther. The designation of the Panzerkampfwagen V Panther only existed from 1 July 1942 through 10 January 1943. The VK 30.01 (H), VK 36.01, and VK 30.01 (P) were all designated the Panzer VI, but the VK 45.01 (H) became the Panzerkampfwagen Tiger Ausfuehrung E.
I should have said the issue was complicated by the Neubaufahrzeug, which were never officially counted as Panzer V and VI, but managed to acquire that designation in the eyes of Allied intelligence, which confused the issue when the Panther and Tiger appeared. Even worse, the Germans delighted in multiple, confusing designations for the same vehicles. They also were designated according to the Sonderkraftfahrzeug designation, plus the prewar cover names.
The Panzer V designation was also applied to a prewar prototype, a few of which saw war service. This was basically a propaganda gimmick, to give the impression that Germany had another tank model operational. Neubaufahrzeug - Wikipedia