Tom has a really good point that in many lines you will find the colour is can be slightly darker when mixing two different levels.
Here is a chart I have saved from way back, and I'm not sure what line it even originally comes from, so definitely don't apply it to your colour line, but I though I would post it to demonstrate why this can happen.
Level - Pigment Weight
10 -3-4 units of pigment
9 -8 units of pigment
8 -10 units of pigment
7 - 15 units of pigment
6 - 25 units of pigment
5 - 40 units of pigment
4 - 75 units of pigment
3 - 100 units of pigment
2 - 125 units of pigment
1 - 150 units of pigment
If you take the maths equation a step further, mixing a level 7 with a level 5 (15 units + 40 units = 55 /2 = 27.5 units) would give you a colour
slightly darker than a level 6. I'm not saying for one minute that you should start using this chart in your mixing, it's just to demonstrate why mixing levels isn't always perfect when you go too far apart.
To help understand mixing different tones, I found that studying pure-tone colour lines such as Aveda really helps to understand. This is a simplified version of how the line allows you to add amounts of pigment at different levels:
If you imagine a 'true' level 8 gold would have 6 grams of gold, and a 'true' level 7 copper would have 8 grams of copper, you can see why mixing tones on different levels will influence the final tonality. It also helps you to understand that the amount of tone in a natural/soft dark colour, would appear as strong or intense if a lighter level had the same amount of tone.
Again I am not suggesting you use this chart with other brands, because they are all buillt differently - it's just to illustrate a point (hope I haven't made it more confusing :o )