Yes, so:
tren lowers systemic/liver IGF-I (because it reduces natural GH release and does the next thing):
but tren dramatically INCREASES the actual muscle's response to that systemic IGF-I (it does this by making more efficient use of that IGF-I as a precursor for special subtypes of IGF-I that exist inside the muscle)
so, though tren lowers natural (endogenous) GH levels, if we force that GH into the body, it'll still increase that systemic IGF-I floating around and allow for more opportunities when that IGF-I that's floating around comes into contact with muscles to increase the muscle's growth.
Even if bloodwork for serum IGF-I is "low," on tren, be sure that tren is making better use of the IGF-I inside the muscles.