The longer yardages are for if you want/can avoid joins/seams in each border strip. Which is my preference when I'm able to do so -
But many times one has to improvise/piece a border for various reasons:
Only so much of the wanted fabric is available
Only so much money to buy the wanted fabric is available
The design of the print makes one direction more appealing than the other
Good luck.
I'm a wash before cutting person. I've found that most fabrics shrink - but I never know ahead of time if it will be lengthwise, crosswise, or both ways. So I always try to have a bit 'extra' to allow for that - and as Prism said - if you do the diagonal seams for the joins - that also uses a bit more fabric.