Well the manufacturer's have their own idea of how their bikes should fit the people that buy them. If they went by the same rules, then there'd be little need for other brands. Similar to how some bike shoe brands fit wide feet better and others fit narrow feet better.
If you don't have a better idea from your experience, then I'd let their advice weigh heavily on your decision of what size to get.
If you are really wanting to see the differences then you need to take the specs for each, draw them out and overlay them so you can visually compare. As well you need to do the same with a size up and down from those.
Still, what do they feel like when you ride them? Even if just around the parking lot you should get something. If you aren't going to be on these bike three or four hours at a time, then it's not too big a deal.
Last edited by Iride01; 07-21-19 at 02:40 PM.