i'm currently shopping for some new tires/rims for my bronco. i've been browsing ads in mags and online, and i'm noticing some sizes are listed differently than others. i used to be a service writer in a tire/service shop, so i'm knowledgable of fitment listings, but i just can't quite come up with the reasoning behind this...

some sizes listed like this-
35x14.50x15
37x13.00x15
which means 35/37" tall x 14.50/13" wide R15.

then i see some like this-
16x35x15
18x39x15
which im assuming is 35/39" tall x 16/18" wide, R15.
whats the purpose behind the mixup?

There's not really a purpose behind it. They just like to screw with people's heads.

Don't forget the sizes that are something like this 205/60 R 15

205 mm tread width
60% ratio between sidewall heigth and tread width

R radial tire
15 rim size