Creeper

we all know that every mob has it's radius of acknoweledging your presence. But, when i move (without sneaking) 2blocks bellow creeper's standing position, with stone fence right in front of creeper, it acts neutral, even as i stand 1block away of that fence (2blocks bellow its standing poss). I do not know why, 'couse every other mob does turn hostile in same situation, but creeper does not (at that situation, its head can see me entirely). When i hit it with an arrow, it will not turn hostile too, even as it starts looking right at me then, but still neutral. This makes it easy-to-kill creeper with stone fence and 2blocks bellow it...