Here's why in noms from Senators may not always be more competitive than those from Reps. Senators generally feel compelled to spread their noms across their state, even if the majority of well-qualified students are in a particular region. It would look bad if most/all of the 10 noms came from a handful of schools or one or two main locations in the state. Thus, someone in a less competitive part of the state may be more likely to receive a nom than the 3rd or 4th best candidate from the super-competitive area/school, even if that latter candidate is more competitive.
In most states (other than those with only one Rep), Rep districts are smaller. Thus, while they too want to achieve geographic and school diversity, it's smaller pool, so that 3rd or 4th best candidate has a better shot.
Obviously, if your state only has one Rep, it's all a moot point since everyone is competing state-wide.
Finally, in some states, there are more nom spots than qualified candidates or even candidates. In those states, it's not hard to obtain multiple nominations. That doesn't mean those receiving noms are necessarily less qualified, but there are so few candidates that there's no real competition.