Where do you get this from? There is nothing in the lore that states you have to be N "enter number here" to be considered for Spectre candidacy. All we know from the codex is that Spectres need to have years of Military or Law Enforcement experience to even be considered, something Ash and Kaidan have in abundance. By the way, using your logic, individuals in Law Enforcement would never be considered for the Spectres since none of them are N "enter number here".
You put way too much stock in this N thing mate.
But if one is going to nominate a soldier, then one should nominate the best possible soldier.
Too much stock?
Interplanetary Combatives Training (ICT) is the Systems Alliance's premier school for leadership and combat expertise. The Interplanetary Combatives Academy, sometimes called "N-School" or "the villa," recruits officers from every branch of Earth's militaries to partake in grueling courses at Vila Militar in Rio de Janeiro.
Initially, candidates train for more than 20 hours per day, leading small combat teams through hostile terrain with little sleep or food. Trainees who do well are awarded an internal designation of N1 and are invited to return. Subsequent courses - N2 through N6 - are often held off-planet and include instruction in zero-G combat, military free-fall (parachuting), jetpack flight, combat diving, combat instruction, linguistics, and frontline trauma care for human and alien biology.
The highest grade of training, N6, provides actual combat experience in combat zones throughout the galaxy. If the trainee survives these scenarios in "admirable and effective fashion," he or she finally receives the covetedN7 designation. N7 is the only ICT designation that may be worn on field or dress uniforms.
There is little shame in failing an N course - the training is so extreme that even qualifying for N1 elevates an officer to a position of respect. The universal prestige of merely attending the academy helps to restrain trainees from taking excessive risks in pursuit of higher honors.
Although ICT qualification by itself does not guarantee higher rank, those officers who are able to complete the program are typically well suited to senior leadership positions.