With TTL flash, this is normal: the camera’s regular meter mainly reflects ambient light, while the flash system separately meters the flash burst and adjusts power automatically.

A simple way to work is:

• Set shutter speed at or below your camera’s flash sync speed.
• Choose aperture for the depth of field you want.
• Let TTL handle flash power for the subject.

In practice:

• Flash exposure is mostly affected by aperture and flash output.
• Ambient/background exposure is strongly affected by shutter speed.

So if your subject is exposed well by TTL but the background is too dark, use a slower shutter speed to bring up ambient light. This is often called “dragging the shutter.”

If everything in the scene is about the same distance, TTL should usually do a decent job. If your subject is closer than the background, TTL will tend to expose the subject and let the background go darker unless you increase ambient exposure.

Because TTL behavior varies by brand and model, the best way to judge the final result is to review a test shot and check the histogram rather than relying only on the meter display.