Why is 'Parliament' spelt without a capital sometimes?

Solution 1:

Parliament can refer to any legislative government body. Its origin is in an assembly gathered to talk.

c. 1300, "consultation; formal conference, assembly," from Old French parlement (11c.), originally "a speaking, talk," from parler "to speak" (see parley (n.)); spelling altered c. 1400 to conform with Medieval Latin parliamentum. Anglo-Latin parliamentum is attested from early 13c. Specific sense "representative assembly of England or Ireland" emerged by mid-14c. from general meaning "a conference of the secular and/or ecclesiastical aristocracy summoned by a monarch."

Typically when referring to the English House of Commons, it is capitalized as a proper noun. When referring to an assembly in general, it is not since it does not refer to a specific proper noun.

Other interesting uses: The collective noun (term of venery) for owls is a parliament of owls.