Nobody is symetrical. Your right leg might be turned out a tad more, or in, or your horse might not be symetrical. What if your saddle is worn unevenly? There are a hundred thousand reasons why the bruise started on righ tright leg, but the fact that it began, and grew, indicates repeated irriation, so its a good be the boot and clothing in the area contributed to it. Or, perhaps a stirrup leather.
In the end, only you know what is rubbing there. You can’t be entirely ignorant of what rubs in that area and when it does so. Take note. Make changes if necessary. Problem solved.
I’ve had myriad bruises and bumps from clothes, boots, shoes, helmets, gloves. Hundresd over the years. From skiing, riding, hiking, playikng polo, swimming competitively. I just never thought to mention them to anyone before, not my family, unless I needed help getting new equipment, but really, never ever thought to ask strangers why they think I have them. I don’t think any of them is a mystery, once I thought about it. Probably yours isn’t, really, either.