Query Plan Different When Using Compiled Values
Most likely, either:


The state of the database (e.g. statistics) changed since the cached plan was first compiled, causing a recompilation; or
Your SSMS connection uses different settings (e.g. ANSI_NULLS) from those used by the connection that cached the plan (so it could not be reused); or
It was an ad-hoc batch and the text of your query did not exactly match the cached version
You used literal values where the original plan was parameterized