Swifts often want less powder than other similar weight bullets. Their copper jackets are "grabbier" in the bore than the more commonly encountered gilding metal jackets.
On top of that the Interlocks have bearing surfaces toward the short end of the scale which reduces pressure.
Combine those two factors and the discrepancy you're seeing is readily explained.
If you're careful working up with the Sierras they'll come out pretty close to the Hornady numbers.
The node you're looking for may not land on the same velocity as you found with H4350. They're located more by barrel time than speed.
