Maxscript - render command memory issue

Hello guys, in my script I've noticed that my 3ds max memory usage stays relatively the same when i write it like this:

render camera:leftCam outputwidth:spn_frameWidth.value outputheight:spn_frameHeight.value vfb:chkbox_vfb.state cancelled: &bCancelled progressbar:true outputfile:fName framerange: (interval spn_rendStart.value spn_rendEnd.value)
render camera:rightCam outputwidth:spn_frameWidth.value outputheight:spn_frameHeight.value vfb:chkbox_vfb.state cancelled: &bCancelled progressbar:true outputfile:fName framerange: (interval spn_rendStart.value spn_rendEnd.value)

however when i try to put it in a for loop the memory builds up significantly

for i = spn_rendStart.value to spn_rendEnd.value do 
(
	if not bCancelled then 
	(
		RenderFrames stereo:chkbox_stereo.state stitch:chkbox_stitch.state
		index += 1
		slidertime +=1
	)
)

in my RenderFrames() it is basically the same as the first except without the framerange parameter.

I've narrowed the problem down to creating a new bitmap inside RenderFrames(). I thought that close() bitmap would solve that memory issue but it still persist. any ideas?