No one knew I was breech until I was 6 cm dilated in active labor. Why was this a surprise if I kept all of my prenatal appointments?
Part of the problem with the whole delivery process is that it is considered a natural event, so obstetricians have to walk a fine line between letting things happen with no intervention versus too much intervention. Certainly insurance companies look at simple ultrasound as intervention and discourage its use as a routine diagnostic tool. Although obstetricians disagree with this type of thinking (we love ultrasound), still we are under scrutiny to order such tests only when there is clear indication. Believe it or not, we are being tracked according to criteria such as complication rates, lab tests ordered, ultrasounds, etc. If a doctor were to fall out of the "norm" as defined by an insurance company's Utilization Review, that doctor may no longer be allowed to see those patients. Designed to manage costs for the consumer, which benefits you, it also tends to get good doctors fired sometimes.
I say all of this only to point out that it is easy to fall through the cracks while trying to practice good medicine on that fine line between not doing enough and doing too much. This is how a breech can be missed.
Also, it entirely possible, although less likely, that your baby was head first ("vertex") and flipped between the last appointment and your labor. Additionally, where the buttocks meet (the "buttcrack," if I may be so crude), can feel just like the loose joining of the skull bones, giving delivery personnel the mistaken impression that the baby is vertex until a wider access like 6 cm makes the truth more obvious because of a better exam.
These things would not happen if we did a jillion ultrasounds with each pregnancy, but these things do happen because we can't or are not allowed to do more than one or two ultrasounds (a lot less than a jillion). We doctors are "jillion-obstetricians" living in a "one or two" world.
An "oblique" position may be nothing more than a baby who hasn't dropped into the pelvis yet--what we called "engaged." Hope this helps.