This is the commentary in the handbook for 312.8
Most enclosures are intended to accommodate only those conductors connected to terminals for switches or overcurrent devices within the enclosures themselves. Where adequate space is provided for additional conductors, such as control circuits, the total conductor fill in the enclosure may not exceed 40 percent of the cross section of the wiring space in the enclosure and no more than 75 percent if splices or taps are necessary.
It appears they are more interested in wire fill than splices. They don't want unrelated wiring junking up the cabinet ... unless there is room.
... unless adequate space for this purpose is provided. 312.8
Splices do not have a fill count.
