Stat EXP is simply taking the base EXP of the pokemon you are fighting, and adding it to your previous stat EXP in that category/stat.
Hence, you can only go up. Sometimes, one thing will raise more, but your pokemon won't be slower if you are facing Gravelers, than say Electrodes.
Unless you are earning EXP, and must stop at lvl 50 for a Cup or something, in which case you COULD have more STAT EXP for Speed, if you had battled faster pokes, but that's about it....
The last remaining Kai Master.
(Email me at Lander_Z@Hotmail.com)