SQLite3 substitution goes sometimes wrong

I have the following code:
    set yesterdayAboveCountC {
        SELECT   COUNT(*)
        ,        :tempAbove
        FROM     temperature
        WHERE    date = DATE('now', '-1 day')
             AND Temperature > :tempAbove
    }
    set yesterdayAboveCountD {
        SELECT   COUNT(*)
        ,        :tempAbove
        FROM     temperature
        WHERE    date = DATE('now', '-1 day')
             AND Temperature > 65
    }
    puts [db eval ${yesterdayAboveCountC}]
    puts ################
    puts [db eval ${yesterdayAboveCountD}]

And this gives:
0 65
################
817 65

So it looks like thet :tempAbove is incorrectly expanded in the WHERE clause in yesterdayAboveCountC.
What could be the reason of this?