I would guess this is because, like all pharmaceutical AAS, it contains an amount of solvent far greater than used in our industry's products - in the case of pharmaceutical Sustanon, it's 100mg of BA per 1ml ampoule of 250mg:
Sustanon 250, 250mg/ml solution for injection - Summary of Product Characteristics (SmPC) - (emc)
Sustanon 250, 250mg/ml solution for injection - Summary of Product Characteristics (SmPC) by Aspenwww.medicines.org.uk
Thanks for the explanation. What is the reason for using more solvents/ml? Can be stored longer?