using 9 of the same runes
 

ive noticed that most people get the same nine marks, glyphs, seals, and quints for their rune pages... is there a specific reason for not getting like 5 flat and 4 per level seals versus 9 per level seals?

Most likely it's your own choice, but i think its because deciding will focus more on Early/Late, for example Flat ad Marks will be a good choice for farming and dominating your Lane early (as ad carry/solo top)

What he said.

Runes and masteries help give you more of an advantage early game than late. There is also nothing wrong with mixing different types of runes (I.e. ad and armour penetration runes or HP and armour seals). It just depends on what champ you are using and your playstyle. Try different things out and find what works best for you.

I've seen more people mix MR runes and decided to do that myself just to save the cost of two full sets of glyphs.

I've seen a guide on a champion, can't remember which, where the summoner put an extra non-red Attack Damage rune in the build. He did the math and worked out that a certain number of auto attacks would leave a sliver of health on a cs (minion).

I've also seen a 6/3 mix of different runes on another champion.

If you're beginning and your last hitting isn't as good as it could be, then having 9 AD reds can help.

Red is Attack, Yellow is Defense and Blue is Magic.

So, flat Attack Damage Reds do more +0.95 damage than AD Yellows +0.43 or AD Blues +0.28.

Whereas Armor Yellows +1.41, give more protection than armor Reds 0.91, or armor Blues 0.70.