Airlines block seats for various reasons - for passengers needing bassinets, for disabled passengers, etc.
Twice now a phone agent has given me the seat blocked for a disabled person, as it was the only MCE seat left - and she made it quite clear that if there was a disabled passenger I would lose that seat assignment.
And even expert flyer isn't completely reliable ... US Airways 321s on EF always show the exit rows as blocked (or occupied), but the blocked ones are fully available to the appropriate elites. I presume EF simply doesn't have enough labels to show the full set of US seat categories.
Even so, EF is often the best way to view a seat map - as most airlines simply show you available / unavailable and EF separates the unavailable into occupied / blocked.