Ive asked about this in rAthena github before and here it is :
https://github.com/rathena/rathena/issues/4700 https://github.com/rathena/rathena/issues/4817
And here the explanation about regarding MVP item drop mode that has been added.
https://github.com/rathena/rathena/commit/5110ddbaf47c358a4a656c32043eb9973b3cd0ab
- Added a new configuration setting "item_drop_mvp_mode"
* By default "item_drop_mvp_mode" is set to official order (0) which is always starting at first, then going to the second if the first didn't drop and only checking the third if the second didn't drop either
* @mobinfo will now properly consider this and display the effective chance, so if the MVP has 3 MVP item drops at 50%, the first item will display the chance 50%, the second 25% and the third 12.5%, this should help a lot against drop rate confusion (e.g. players wondering why always only the first item drops when all three are displayed at 100%); this only applies to official order (0)
* When "item_drop_mvp_mode" is set to random order (1), MVP item drops are checked in random order, but if one MVP item drops, the process will be canceled, so you will never get more than one MVP item no matter how high the drop rate is
* When "item_drop_mvp_mode" is set to all items (2), all MVP item drops are checked and have a chance to drop, regardless of other MVP items already being dropped or not (i.e. there is a chance that the player gets all 3 MVP items)