The epsilon value used to break a tie is computed as 1 over the query knob constant for the max number of works to perform during plan ranking:

https://github.com/mongodb/mongo/blob/r3.1.0/src/mongo/db/query/plan_ranker.cpp#L209-L211

// Just enough to break a tie.
static const double epsilon = 1.0 /
     static_cast<double>(internalQueryPlanEvaluationWorks);

During plan ranking, however, the actual number of works is the max of internalQueryPlanEvaluationWorks and some fixed fraction of the collection size:

https://github.com/mongodb/mongo/blob/r3.1.0/src/mongo/db/exec/multi_plan.cpp#L199-L200

numWorks = std::max(size_t(internalQueryPlanEvaluationWorks),
       size_t(fraction * _collection->numRecords(_txn)));

The consequence is that the tie breaker can be too large. Instead of breaking a tie, it can cause an inferior plan's score to exceed a superior plan's score.