Have no idea about spinervals as I don't use them and when I do train inside I put on a DVD of one of the televised tours.

For me, my riding up hills at a sustained steady pace does not lift my heart rate as much as when I do do sustained outputs of power on the flat... thats when I push my lactate threshold.

So the fatigue I have after both types of rides is different - you use your muscles in different ways. I suspect that is what you are noticing.