Enthalpy and Bond Dissociation Energies
Tabulation table such as the ones you reference, describe 'typical' bonds when using generic groups such as R.  You are right in your discomfort that the numbers should be somewhat different, but that would require a thermochemical measurement for every known molecule, which is difficult to achieve in practice.

Calculations such as the one you are performing are 'estimates', and while good ones, they are never as credible as an actual experiment.