When it arrives at Gate 73 at SFO 20 minutes early from SAN, but UA 294 to BWI is delayed 74 minutes due to "late arrival of equipment". Agent at gate says with straight face, when plane is clearly there EARLY, that the delay will be due to late arrival of aircraft. Called 1k desk for their take - mechanical. Pilot said it was windy - but everything else took off on time. Sometimes - they don't make it easy!