I can't answer why your shbg is low but with shbg that low, you probably don't need more test. I'll bet your free test is high, probably out of range or just in range.
In natural people low shbg seems to be associated with diabetes/insulin resistance. I don't believe that's why guys on exogenous test have low shbg. I think it has more to do with the way our bodies handle/process exogenous test. I think some of us, myself included, don't handle test very well or our bodies just don't like elevated test levels and in an effort to eliminate excess test, shbg drops to free up test and quickly convert it to estrogen and dht.
Or possibly for some of us, low shbg is a by product of being shutdown. Maybe shbg goes down to compensate for the lack of natural test production despite exo test.
With shbg at 15 and estradiol at 29, do you feel like you have high estrogen symptoms?