The 100% battery health displayed from a new iPhone is almost never actually 100%. 100% is the baseline/minimum capacity all iPhones of that model come with. In your case, it was likely one of the few that didn’t come with say, 5% bonus capacity. Battery degradation is expected behavior. The SE also has a tiny battery and also relatively short battery life. It’s also powered by a flagship processor in a small chassis, so heat builds up more than it otherwise would, even if you don’t notice when it does.

Also, if you NEVER let the phone fully charge or discharge, the BMS may eventually report an incorrect figure in battery health. The memory effect on current battery tech isn’t as bad as older chemistries, though. Apple allows iPhone 15 series devices to stop charging at 80%, but the new phone will periodically charge to 100% to maintain accurate readings. This could explain why your phone would tell you it has less than 100% capacity. Again, 1% in 7 month is normal, especially if the device sees regular use. Many tech reviewer devices do not, and so they stay at 100%.