Variable scope - database.query() has access to variables not in scope

I was answering a question and this little inconsistency popped up. Just wondering if anyone has come across it before, or if anyone knows why this happens.

A variable that is declared within some form of scoped element (if, for loop, etc.) shouldn't be available outside that scope.

i.e.

{
    // accountNames variable is not available outside this scope
    Set<String> accountNames = new Set<String> {'Test Account'};
}

system.debug(accountNames);

This debug statement fails with the message:

Variable does not exist: accountNames

However, if this is part of a query string being generated, the Database.query() call somehow has access to it.

String queryString = 'SELECT Id, Name FROM Account';

if(true)
{
    // accountNames variable is not available outside the `if` statement
    Set<String> accountNames = new Set<String> {'Test Account'};
    queryString += ' WHERE Name IN :accountNames';
}

// this debug will fail if you uncomment it
// system.debug(accountNames);

// however the query somehow has access to it.
// create an account called Test Account to prove it.

system.debug('queryString: ' + queryString);
system.debug('queryString result: ' + database.query(queryString));

But, Salesforce is smart enough to know that it's not just some random string, so the following will fail:

// Alternatively, use a random string as the variable name, and salesforce 
// is smart enough to know it's gibberish
String queryString2 = 'SELECT Id, Name FROM Account';

if(true)
{
    queryString2 += ' WHERE Name IN :randomString';
}

try
{
    system.debug('queryString2 result: ' + database.query(queryString2));
    system.assert(false);
}
catch (QueryException qe) 
{
    system.debug(':randomString is gibberish: ' + qe.getMessage());
}

Edit:

Interestingly, the :randomString is definitely calculated only when the query is executed.

String queryString3 = 'SELECT Id, Name FROM Account';
String queryString4 = 'SELECT Id, Name FROM Account';

if(true)
{
    // assign to query 3 before it has a value
    queryString3 += ' WHERE Name IN :randomString';

    // assign to query 4 after it has been declared
    Set<String> randomString = new Set<String>{'Test Account'};
    queryString4 += ' WHERE Name IN :randomString';
}

// both queries return correct results, regardless of when the variable was declared
system.debug('queryString3 result: ' + database.query(queryString3));
system.debug('queryString4 result: ' + database.query(queryString4));

But it's not an open slather. The following throws an error:

String queryString = 'SELECT Id, Name FROM Account WHERE Name = :someField';
someMethod();
system.debug('queryString result: ' + database.query(queryString));

void someMethod()
{
    String someField = 'Test Account';
    system.debug('someField: ' + someField);
}

Does anyone know why this is the case, or have an explanation why this would work?

As far as I can tell, at the time that the accountNames variable is in scope, no part of the code knows that it's anything other than a string.