I see two possible reasons for your situation. One is that the company is turning to contractors to fill in gaps in their knowledge/experience, which is why everyone else has no clue how to tackle these tasks and why they get assigned the easy ones.

The other possibility is that the senior devs are gaming the metrics, letting the employees knock out easy tasks while the contractor is stuck with untangling the knots of the more intractable tasks.

I still feel the Perfect Strangers theme fits better.

“Riker!”