Open Sans is a problematic font. There have been no ends of reports of issues with it on this forum.
The designers didn't assign proper style flags internally so Open Sans Italic has the same Style name as Opens Sans Light Italic.
For the Light Italic, it should say "Light Italic". So with both given the same Style name, Your ID is finding the first "Italic" it finds, and it's not necessarily the correct one. I just did a test with it myself and when I applied Italic style, I got Extra Bold Italic instead!!