It really can. EGL grades more generously than GIA. It’s virtually guaranteed to be at least two grades above GIA, but I’ve heard of at least one case where it was 6!
Example of two grades generous:
GIA will say: J color, SI2 clarity, Very good cut.
EGL might say: I color, SI2, Excellent cut
OR, EGL might say: J color, VS2, Very good cut
(and so on. You have no idea where they were generous, it could be anywhere.)
And if it’s SIX grades off (and you have no way of knowing whether it’s 2, 3, 4, 5, or 6 grades off, except by your own eye and knowing well what the color/clarity/cut grade should be), you might get something from EGL (about that same GIA J/SI2/VG):
“G, VS2, Excellent cut”
Now, you might look at the price and think “hey, that’s an awesome deal on a G VS2 Excellent!” and it WOULD be, but that’s not what you’d get… and it won’t be such a great price at all for the J/SI2/VG you ACTUALLY get, with that flattering but entirely inaccurate paperwork.
Also, jewelers can make loose estimates of how their diamond will get graded by GIA. They know that the best way to sell their high quality stones is with the credibility of a GIA cert (or an AGS cert), and they know the best way to sell their lower quality stones is with the flattering extra boost of an EGL “cert,” and they will choose the lab to send it to accordingly. (And their very low quality stones won’t tend to have a cert at all… the jeweler will happily guesstimate it for you though…with a little helping of generosity of course.)
As you can probably guess, my view of EGL is that it’s hardly worth the paper it’s written on! I’m a totally unqualified diamond nut with a good eye and zero credentials, and I’m confident that even I can perform more reliably and fairly than EGL. And you wouldn’t want me doing your grading.
BTW, it is said that EGL-USA is the better of the EGLs, but, for such an expensive purchase as a diamond, the “better” EGL is still too generous, still in an unpredictable way and amount, and basically, still not worth your time, IMO!