Help me make sense of this dialogue from 'The Importance of Being Earnest'

From Oscar Wilde's 1898 play, The Importance of Being Earnest:

Algernon. Oh! . . . by the way, Lane, I see from your book that on Thursday night, when Lord Shoreman and Mr. Worthing were dining with me, eight bottles of champagne are entered as having been consumed.

Lane. Yes, sir; eight bottles and a pint.

Algernon. Why is it that at a bachelor’s establishment the servants invariably drink the champagne? I ask merely for information.

Lane. I attribute it to the superior quality of the wine, sir. I have often observed that in married households the champagne is rarely of a first-rate brand.

-- SO. This Algernon is a batchelor (a man who is not married but is living on his own). When Algernon asks why the servants prefer to drink champagne over wine, Lane answers that wine is better than champagne, which makes it more suitable for servants. But why, in the last sentence, Lane says that "in married households champagne is rarely of first-rate brand"?

This makes absolutely no sense to me. Help?

EDIT: I didn't understand that they were talking about the same thing (champagne, which Lane refers to as "the wine").

At one level, the humor of the exchange lies in the fact that Algernon (a well-bred but impecunious and somewhat dissolute bachelor) asks Lane (his manservant) what would seem to be an irritable rhetorical question about why servants drink their employers' alcohol, but he frames it as a non-accusatory inquiry motivated by simple curiosity—and Lane then responds in suitably analytical terms, as though it were indeed purely a sociological question and not an implied criticism of Lane for drinking the champagne (which, as Brian Donovan point out in a comment above, he may not have done).

At a second level, the author is making a joke premised on the notion that married households tend to keep wine of such inferior quality that their servants are not tempted to steal it.

This exchange shows the level of intimacy between Algernon and his servant Lane. They are joking together, and they both know it, but they have lived together so long that they talk in a kind of code. I offer a translation of the subtext. Algernon. Why is it that at a bachelor’s establishment the servants invariably drink the champagne? Translation: Did me and my mates really drink over 8 bottles of champagne? You must have drunk some. I ask merely for information. You know I'm not really accusing you of drinking it, I'm just kidding. Lane. I attribute it to the superior quality of the wine, sir. I have often observed that in married households the champagne is rarely of a first-rate brand. I wish I had drunk it! You bachelors can afford the best stuff. Apart from being hilarious to the audience, this exchange establishes that both Algernon and Lane are content with their domestic arrangement and Lane has no interest in Algernon marrying. Marriage is a major theme of the play.