It's plain profit, the business model would not work if they sold a 290, 310 and 390 all at the same price as it costs the same to make them. There would be no point in even making a 290 or 310.
I'm just sayin' the 390 is a poor seller compared to the 290. I think Stihl could make more money if they dropped the price on the 390 a bit to entice more customers to make the step up to the higher-profit model.
The 210-230-250 series makes more sense in the pricing to me. Each gap is, what, an extra $30 or $40? That's an easy step to make. And you step up 0.5 hp each time. So do you want a 2.0 hp 210 or is a 3.0 hp 250 worth an extra $70..? I would say that the higher-profit MS250 handily outsells the low-margin MS210... exactly what I would want to do if I were running the company. Sucker them in with the low-margin product and then upsell...