Marth's fair has so many different knock backs

[Sprenzy]

depending on the percentages, character etc it's so hard to have complete control of fair
for example you can have your fair knock back at slight downwards angle, you can fair so that the opponent goes up slightly, etc. does anyone has a chart that shows these things?

 

[Stride]

http://www.ssbwiki.com/Marth_(SSBM)/Forward_aerial

 

[T^2]

the difference you're seeing I think is the difference between a tipper fair and a non tipper fair. the non tipper has less hitstun and does the downards angle, while the tipper does more damage (I believe) and knocks up at a slight angle. Tipper is if you hit someone with the point at the far end of the sword, the other is if they get hit by the rest of the blade at all.