My guess would be due to premultiplication. Essentially in Maya, the semitransparent parts of your image are being multiplied by the color channel. You can choose to turn that off in the Maya render settings. Better yet, in after effects, for example, you can choose how to interpret the alpha channel, so you should choose premultiplied against black. It's a confusing subject, for sure, and there's lots of reading material out there, so I won't try to bumble through it much more than my simplistic explanation.