It could be mixing of the paint in the bottle isn't complete but Tamiya mixes easy enough. It could be the base color below the color coat you are putting on is different. It could also be different thinning ratios used. You need to be consistent on all these points to get "the same" or repeated result.
That said, the preferred thinning agent for brush painting Tamiya acrylic is retarder not thinner.I know people use all sorts of different things to thin Tamiya acrylic in brush painting and can claim success, just if you go to the Tamiya web site you will find them saying to use retarder. I had no success with brush painting Tamiya acrylic till I did just that. Of course Tamiya recommends their own product but I use Liquitex because it's the only acrylic retarder I ever stocked and has worked in every acrylic paint I've used thus far. That's not a plug for Liquitex it's just what I personally have used for several years in various capacities, from acrylic pourings, brush painting and slowing agent in airbrushing.. I use it in my home brew thinners for airbrushing as well.
Edit for added info: Also it's best to use lapping strokes when brushin Tamiya vs over lapping strokes. With lapping stroke you just brush one stroke against the next and they will flow together using the retarder as thinner. Where with over lapping strokes the second stroke can disturb the first stroke causing uneven layer thickness. Just sayin.