I'm afraid it is simple supply and demand and competition. Ventura is in competition with other family ships in the RCC, MCS and NCL lines. Azura etc ire more in competition with more expensive (some would say better) lines such as Celebrity.
If I were you I'd look at Royal Caribbean, Celebrity, Fred Olsen etc.
The answer bears NO resemblance to the question,maybe you should explain yourself.
Check out the prices for P&O on this site. I must say in my experience I have managed to get good deals on all ships in the P&O fleet and have recently booked A cruise on the Azura at a very good price for September. I have never noticed much difference in price for similar cruises.
Ventura is a one-off ship for P&O in that it specifically caters for 25-45 year olds with children, having Noddy and Big-Ears on board for example. The trouble is, that this was not made fully clear by P&O when it came into service 3 years ago; and a huge number of regular P&O cruisers, many of advanced years, went on it simply because it was new, and then naturally disliked it because it was so different. The adverse publicity has dogged it ever since, although it is now getting a lot of very favourable reviews from people who gone on board with no pre-conceived ideas. However, I think that P&O are still finding Ventura a bit hard to fill week in week out, especially during this recession, given that a lot of older people still don't like it, but then it was never intended for them anyway. P&O fares look expensive, but the cost of wine/tips/etc; are far less than those cruise lines with cheaper fares. Hope all this is of interest to you, Hugh.