I’m going to write a lot of text now but basically the TL:DR of it is what you are doing is not good practice for image analysis and it would be much better if you would measure the pixel intensity values of your original microscopy image. Please let me know if I don’t explain anything well enough.
When you convert to an RGB image, a pixel value is assigned to the red, green and blue components of the resultant image. In your case, probably what is happening is the intensity values in your DAPI channel are being assigned to the blue component, the 488 channel to the green component and the red component is full of zeros. When you measure an RGB image in FIJI and getting a mean intensity, the value you get is the sum of those three values divided by 3.
Then, when you are converting to composite you will have your three components split up, including an empty red channel. When you measure now, you will get a different result depending on if your channel slider is set to the red, green or blue channel. In your picture, I can see you are on the green channel. This measurement is there for the pixel intensity value assigned to the green component during the RGB conversion in LAF. If you moved the slider to the right you would get the blue channel measurement.
As an extra complicating factor, if your image was captured at something higher than 8-bit (quite likely I would say for most microscopes these days) then there is also scaling happen to make it 8-bit to get it into RGB. This whole conversion process is destructive, resulting in data loss and so it’s likely both of the measurements you are taking are inappropriate for image analysis.
I can see that you are using FIJI which automatically has Bio-Formats installed, this means you can open your .lif files without needing to export to .tif format at all. And this is what you should do really as it’s the raw data from your microscope. So I suggest you try that.
If you are having a problem with inputting LIF files into FIJI then please let us know what the problem is and we can try and help.