When I'm booking a family trip for the 5 of us I sometimes find it to my financial advantage to make two bookings. For example for 3 PAX i'll get a €100 fare and for the remaining 2 I'll get an €120 fare giving a total spend for 5 passengers of €540. If I had booked 5 passengers together all 5 would have breen offered €120 each giving a total spend of €600.
For long haul travel I booked directly with Singapore Airlines AKL-DUB. Fare offered routed us via SIN and LHR. However their booking engine offered what transpired later to be too tight a connection time in LHR resulting in a missed connection and an overnight in LHR so in that case the web site probably wasn't 'fair' to the consumer in offering such a short connection time.