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Let’s say you have a strongly emitting Ha region with no reflection nebulosity, no need to name one as there are many out there. Luminance and R channels will be picking up the Ha whereas green and blue will have no data in those Ha regions excepting some weak OIII emissions. If you remove 33% of the R channel from the L it will still leave some strong Ha emissions as you haven’t removed all of the Ha. Removing 33% of the green won’t reduce those Ha emissions so your resulting synthetic blue channel will be a strong Ha and half strength OIII emission channel.
If the L filter covered exactly the RGB emission lines and there was no overlap and the cutoffs were perfectly clear (straight lines) then you can scale the R&G channels in relation to L and then subtract all of the R&G channels from the L to leave a synthetic B.
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