Testosterone Cypionate takes 48-72 hours to hit peak levels after injecting. Once per week injections means that you're hitting your peak a couple of days after your injection and a decline over the next 7 days. Test cyp has roughly a 7 day half life.
Increases in estradiol coincide with increases in testosterone and will hinder performance in the gym. Need a quality aromatase inhibitor to control that.
Also, as ACES said, your levels are determined by when the blood was drawn, so if you drew at, let's say, day 6, you will read "low" in terms of what is optimal levels. Every other day injections allow for more stable T levels in terms of troughs and peaks. I do the T/hCG/AI all at the same time EOD. Some say estradiol is easier to manage on EOD injections as well.