Yes, "feminized" seeds can produce male plants, and often produce hermies.

7 out of 500?? Something is definitely wrong. Are you certain of your sexing skills?

Plants that are stressed when young often turn male. Could it be that something in your set-up or regimen is stressing your plants?

Also, excess N will masculinize plants.

This link discusses several methods for increasing the number of female plants:
http://www.rexresearch.com/hhusb/hh4bot.htm#HH49

And then there's the birth control pill thing... Soaking the seeds with a pill before sowing is said to turn them female. I've never tried it, so I can't say how well it works, if at all.

Better luck to you!