This is just my hypothesis and not based on any scientific knowledge but here goes anyway:

The hair is a shaft that light can pass through and along.
But your real hair has one end sunk in to the scalp, unlike replacement hair which has both ends exposed to light, which can then pass all the way through it, like an optic fibre thread.

The light refraction makes it appear different to your real hair.

Was that a load of rubbish or perfectly plausible?

HairFanatic

Using some sort of product will definitely help.
There are lots that matt the hair and even more that gloss it.
The trick is judging how much to use on your own hair and how much on the replacement hair.
If you play around with using different amounts, you'll soon get know what works for you.
Remember your natural hair is constantly being fed oils etc which the replacement hair doesn't benefit from.