strange behavior of -density

Hi there,

I use `convert' to convert some figure in tiff format (in 300dpi) to png
with lower resolution:

convert -density 30 -resize 10% input.tiff output.1.png

and I get the output.1.png with resolution 29 dpi. it seems right.

However, if I convert the original tiff to png with the same resolution, and
then convert the png to lower resolution png or tiff, I get different
resolution.

convert input.tiff output.2.png
convert -density 30 -resize 10% output.2.png output.3.png

and I get the output.3.png with resolution 76 dpi.

If I convert the output.2.png to tiff format,

convert -compress none output.2.png output.4.tiff

and then

convert -density 30 -resize 10% output.4.tiff output.5.png

the resolution of output.5.png is 76 dpi.

I don't know why the resolution of output.3.png and output.5.png is 76 dpi.
Any cues, comments or suggestions? Thanks!

P.S. the original input.tiff is obtain from GIMP with no compression.

Regards,
Jinsong