It looks like people can't into math. Or they're in for the other benefits of full shares, of course.
From Havelock, yields:
AM1 32.76%
AM100 33.53%
In the near future AM1 shares should be convertible to direct shares. Maybe that explains the slight difference. AM100 shares should be multiplied by 100 x (100/95) for comparison as the 5% fee is priced in. That currently gives 3.789 vs 3.9 for AM1.
There will be additional buying pressure for AM100, since it's easier to get into as you don't need almost 4 BTC to get a single share. This makes AM100 much more attractive for smaller investors, which translates into increased demand.