Divergence (spoilers)

Maybe I missed something, but, if, as explained by Katie, the pen will only roll one predictable way because all of the data/circumstances/reasons leading up to the pen being pushed are consistent, and everything after the rolling of the pen can also be predicted based on all of the events/factors/reasons up to the point at which it was pushed, and as we find out later that no one has any choice but only acts based on those same factors, and their behaviour can be predicted 100% reliably, then surely there is no possibility that before Lily, for the first time ever, makes an actual free-will choice (at which point there are now two worlds, not many), there could be more than a single thread of events leading up to that point?