5e has very, very tight accuracy math that intentionally does not take into account the effects of +x magic items. A character with a +1 sword will hit 5% more often than they are “supposed to”. A character with a set of +1 armor will be hit 5% less often than they are “supposed to.” This is by design, and was done mostly in response to critique that, in 3e and 4e, you need certain +x items by certain levels just to keep up, which meant if the DM didn’t give out the right items on the right schedule, the party could end up too weak (or too strong, if they gave out too much) for their level and break the CR math. It also made magic items an expected thing on the player’s side, which some folks felt made magic not feel rare or special enough. So, for 5e, they decided to leave magic items completely out of the encounter math.