The only thing that strikes me as weird is that the result would change if you change the order in the add node (because the factor is 1).
The point is that you should not interprete the add node as putting two layers over each other. Instead, you have to see it as adding two numbers. Each pixel in your image has a RGB value. The add color node creates a new pixel by just summing up the values of the same pixels in the two inputs.
This means, for your first line of text, which is white and thus has an rgb value of 255,255,255 (the maximum), adding something up doesn’t matter. Because you already have pure white. The order of adding up shouldn’t matter: 2+3 = 3+2, right?
The second line is blue (0,0,255). Adding the gray lines (100,100,100) would result in (100,100,355). Now 355 is to high and blender can’t show that, so he shows 255 instead (if you select the clamp option, he will actually change the value instead of just showing it differently), so here the lines do have an effect.
In my case, I have the same effect, however, I changed the factor to 0.1. This will change the second input (so now the order does matter) and (100,100,100) becomes (10,10,10), adding it up with the blue: (10,10,265). The difference with (0,0,255) is so subtle you hardly see it and that is why I left it the way it was.
Something you can do if this effect keeps bothering you is creating a mask: add a less than node and set the value of the second input very low (almost 0). The first input should be the raw text image. The result of this will be a black and white image with only your text black and the rest white. Feed this into the factor amount of the add node (with the lines as second input). The lines will now be multiplied with the values of the mask (black = zero where the text is and white everywhere else). Adding a multiply node between the less than node and the add node will allow you to controle the factor of the lines again.
Does this help?
PSI: Only read this once you understand the above: I used 255 for white in my explenation, but usually Blender will rescale that between 0 and 1, which is easier to calculate with. In this case, blue would be (0,0,1). It is the same, just using a different scale, but it is perhaps better to think of colors between 0 and 1.
PSII: If you ever do want to combine two images as if with layers like in photoshop, there is the alpha over node. Just make sure that the top image has an alpha layer or you won’t see the bottom one.