The impedance can be calculated at any point along the signal path. In this manner the designer will know ahead of time if one is meeting the 50 Ω. Therefore the RF signals look at the distributed impedance along the signal path. So when the RF comes upon a change in impedance (say 55 Ω instead of 50 Ω), part of the signal where the impedance has changed will be reflected backwards. This translates into an actual loss in signal strength.