Beef – help me identify the factors which make grass-fed beef tender or tough

I have been buying grass-fed beef straight from the farm both in Germany and in Poland. The beef I bought in Germany was invariably tender, regardless of the cut and what way I prepared it, while the beef from the farm in Poland was usually tough, chewy, to the point of being unusable for anything else then making broth. Even the supposedly premium cuts where somehow dry and tough.

Summary of all the possible differences:

1. German beef was from Highland breed cows, Polish beef where Salers, Piedmontese, Limousin.
2. German beef was hanged for two weeks after slaughter, Polish beef was cut into pieces almost immediately after slaughter, and sold to me within 3-5 days.
3. The beef from Germany I mostly ate before freezing it, the beef bought in Poland I have mostly frozen, and kept in the freezer for up to a few months, -32 deg C. Some of the German beef I also frozen, but for a much shorter time (days). Still very tender.
4. The Polish beef was slaughtered about 24 months old, the German I am not sure, might have been a little younger, but not much, maybe 15 months old.

I was buying all kinds of cuts, both cheap cuts and premium cuts. Even the cheap ones from the German beef where quite tender after slow-cooking them a few hours. The beef from Poland, regardless of how I cooked it, fast or slow, where somehow tough, chewy, dry. Even the Entrecôte from the Polish beef, was much tougher and drier then some cheap flank from the German Highland beef. Both the German and Polish beef where purely 100% grass-fed on pasture. The only tender meat from the Polish beef was when I slow-cooked brisket on the bone overnight in broth. And the tongue (also cooked overnight). But the German beef, even cheap cuts I can just braise for an hour or two, and its amazing.

My question is what is the most likely the major factor in such astonishing difference between the two sources of grass-fed beef?

Is it just the breed, and that Highland cattle is more juicy and tender?

Does the two-week hanging of the whole steer make the difference?

Or does the long-term freezing of mine destroy the beef, freezer burn?

I don't think its the way I cooked them which made the difference, as I tried both beef in several kinds of ways, mainly braising, slow cooking, crock pot, cast iron dutch oven. Never frying or anything like that.

I was buying the beef from Germany shortly in 2013 and then in 2014 I was on the Polish beef and I though I just had luck in 2013 to get a really tender steer, but now in 2015 I bought from the same farm in Germany again, and again its so tender and juicy compared to the Polish Salers, regardless of how I cook it.