The reason they sound different is because they are two different horns. Neither one is a K5LA. EMD uses a K5LLA on the SD70ACe. GE uses a K5HL on the Evolution Series.
K5LLA info:
http://atsf.railfan.net/airhorns/k5lla.html
K5HL info:
http://atsf.railfan.net/airhorns/k5hl.html
Also, there are some variations from horn to horn, due to manufacturing inconsistencies, which can result in two horns of the same type sounding quite different. On top of that, they were some slight changes made to the K5HL after 2007, which means that the horns on older Evolution Series locos sound different than the horns on newer Evolution Series locos, even though they both have the same type of horn.