I have a Sunpak PF30x on a Nikon D40. When I take pictures with the Sunpak flash, I notice the image data has "Optional-TTL" as the flash mode.
Optional-TTL means you're using an accessory flash instead of the built in flash, but still using the through the lens metering to control it.
Maybe because of how your model is setup?
This flash unit supports flash exposure compensation which allows you to adjust output independently of the i-TTL.
Disclaimer: I don't have one of these units. Just what I noticed from a simple websearch about this product.