Why is azitromycin sometimes ineffective for treating rectal chlaymdia? what's the biggest difference between that and deoxycline?

Rectal chlamydia: In animals, azithromycin works less well against chlamydia in the gastrointestinal tract than in the genital tract. It is likely the same in humans, explaining the higher rate of treatment failure for rectal than genital infection. The biological reason is unknown. This appears to not be a problem with doxycycline, which works equally well for rectal and genital chlamydia.