There are 2 737-800/900 one has very shiny chrome around the opening of the engine cowl like it was just polished and burnished, the other is dead flat grey.
Generally, it just means that one has been polished recently and the other hasn't.
I'm pretty sure it's a different nose cowl/nacelle for improved laminar flow. Later 737 deliveries have the matte finish while you'll see earlier 737s clearly sporting a chrome look, even if not recently polished.
I used to work at a Heavy MX Base for CO, and I can tell you that the grey matte-finish engine nose cowl is what was and continues to be delivered new from Boeing.
New planes arrive that way, and their inlet cowls are very slick to the touch.
However, time in use causes the nose cowl finish to abrade, especially the leading edge.
When the cowl goes through an overhaul, there is a chromium deposition process that builds up what has been lost and makes it very shiny -- it is not a burnishing of the original surface, which would remove more material, but rather a replacement of lost material with a hard chromium coating, subsequently polished.
This may merely be a choice by United / Continental on how to have these 737 parts overhauled -- other airlines may do something else entirely.
So -- you can tell that an engine nose cowl on a 737 is older by its shiny chrome finish -- and know that it has been through an overhaul.