this is basically the gamble anyone takes when ordering from zenni/other online retailers - the measurements and adjustments that the opticians make at the eye doctors office create a custom fit for your glasses. When you order a pair online, a lot of this process can’t be performed to the same standard. For most people with low/relatively normal prescriptions, this doesn’t really make much of a difference. But for those with larger amounts of refractive error, small adjustments can make a big difference and it can be hard to get that right online.