The difference is in what is known as "first pass". Oral medications must first pass through the liver before getting into the bloodstream. This means that higher doses are required to achieve optimum levels in the blood.
Injections and even patches bypass the liver, so lower doses are more effective at maintaining those levels.
I had issues with oral and switched to patches. My levels continued to be too low. Between my Primary doctor, Endocrinologists, and Gynecologist, we figured out that I metabolize hormones very quickly. So my body was processing and eliminating them before they could reach optimum levels.
Now that we figured that out, I was switched to injections. So far, three lab tests show my levels are exactly where they need to be. I have another lab appointment tomorrow, so we will see when those results come back in a couple of weeks.
I hope this information is helpful.