What network configuration would cause inconsistencies in the BSSID while the source address varies between frames?

In an Independent Basic Service Set (IBSS), devices communicate directly with each other without the need for an access point, forming an ad-hoc network. When multiple devices are part of an IBSS, they each generate their own Basic Service Set Identifier (BSSID) based on their MAC address. Because of this decentralized nature, different devices can initiate communication and have varying source addresses for their frames, leading to inconsistencies in the BSSID.

This is a fundamental characteristic of IBSS, where the absence of a central point managing connectivity results in each device contributing to the network with its own unique identifiers, thereby causing discrepancies in BSSID across the frames transmitted.

In contrast, in a BSS involving a single access point, you would expect consistent BSSIDs as all frames would share the ID of the access point. A mesh configuration typically maintains consistent identifiers across nodes to facilitate routing, and repeater networks extend the original BSSID rather than introducing variations. Thus, the nature of an IBSS inherently supports the situation described in the question.