2002.03.07 20:15 "Re: TIFF LAB with JPEG compression", by Tom Lane
libtiff saves non-rgb images with Y compression for all channels. The jpeg Y compressor does not know that it needs to be very careful around value 128 (a/b == 0), so compression noise is moving things between red and green.
You cannot realistically expect that that range of values will be represented more accurately than any other range. If that area seems particularly sensitive to you, perhaps you should rethink the choice of color space.
But possibly the answer is just that Q 75 is too low to meet your quality standards. Try something around 90.
regards, tom lane
organizer, Independent JPEG Group