Fard or gharib asks how many transmitters occur at a relevant point. Shadh asks whether a narrator contradicts a more strongly established report. Thus singularity alone is not evidence of a technical anomaly.
Case 1: A trustworthy, highly precise narrator is alone in a report, with no reliable contradictory transmission. His isolation does not automatically make the report shadh.
Case 2: A narrator gives an addition incompatible with more strongly preserved routes, and the conflict is established after comparison. That particular addition may be anomalous even if its narrator is generally trustworthy.
Thus counting routes and assessing contradictory evidence are different tasks. A singular report can be sound, whereas one specific report from a normally reliable transmitter may still prove anomalous.