Note that the checks to see through your ruse are in reference to using the ability at all, not to specifically mimicking an individual.
My interpretation:
-Your Changeling alters their appearance to that of another race. Not any specific person, just a new identity for themselves. They do not need to make any Bluff checks to convince everyone who simply sees them that they are in disguise - instead, just walking down the street, no one sees them as out of place in any way. If someone does get suspicious of them, and takes a careful look to figure out if anything is wrong with them, then they make the Bluff check (with the +5 bonus from Changeling disguise) - they fail, and the suspicious person sees through their ruse and realizes they are dealing with a Changeling.
-If you are copying a specific person, and encounter people who would know that person or how they would act, that is when you start making your own Bluff checks opposed by their passive Insight. This represents an active attempt to deceive them and play a part, which is separate from simply disguising your natural form.
While not 100% sure that is how it is intended to work, it would seem consistent with the language of the power, and does seem an appropriate way to deal with such scenarios by the 4E rules.