As crazy as it sounds, i think it has a lot to do with the person as well. I used to run a 9660 that got me about everyday. Yet, another guy who ran that combine fairly often as well never got zapped once. Explain that...?
I'm thinking it may be related to what you are wearing. I have el-cheapo nylon seat covers on my ute, and during harvest I wear those el-cheapo Hiviz shirts. Static seems to build up when the two rub, and I always seem to get a boot when I get out and touch the door to close it. In winter when wearing cotton shirts I don't notice it.
Installing a drag chain will discharge static from the machine, but not from you, as you are insulated by the rubber on the steering wheel, the cloth on the seats, the plastic on the joy/gear stick, and the rubber on your boots. Touch something metal an then you discharge.
Having said that, I've never got a boot while getting off a combine, even with the nylon Hiviz