The reason why the variance is lower is the effect of jackpot hands.
If you're drawing to RF4 on 3-line, you'll get some results of no royals, some results of 1 royal, some of 2, and perhaps once in a lifetime, all 3.
When you draw to RF4 on 3-line Spin Poker, the only possible results are 0 royals and 1 royal. Because there are no repeated cards on screen. You have 3 interdependent chances at the missing card (1 in 47, 1 in 46, and 1 in 45) rather than 3 independent chances each at 1 in 47.
Spin Poker variance is higher than regular play for all cases above 3 lines. Because a drawn card triggers multiple paying lines. But for exactly 3 lines, it's lower. (Someone else can link to the Wizard Of Odds page that gives the exact differences).
Math challenged here. Does it also follow then that playing 5 lines of 9/6 JOB spin poker will have a slightly lower volatility (variance?) than regular 5-play?