Guideguide and PS CS6 false gutter width

Question

Hi
I create a new  project with this setting
canvas size  width 1060 pixels /  height 2000 pixels 
image size   width 1060 pixels /  height 2000 pixels
the i install guideguide and i set the value find in the attach upload psd  to create a vertical guides refer to 960 grid  .
i import 960 raster grid in background to be sure than the vertical guide are ok
But there is a problem when i want to verify that the width of all gutters is 20 px manually , when i use the rectangular marquee tool  ( that stick to the vertical line guide ) photoshop show me for gutter G1 and G10 the value 21 pixels width !!! In a way photoshop marquee tool don't succeed to begin drawing  the rect selection using the vertical line create by guigeguide for some gutters /column ? !!
the first and last column width is also set to 61 rather than  60 px  !!!.
Does the error come from photoshop , guiguide  or windows 8 64 bit ?
Is there a setting to take care to have not this problem ?
regard's
http://cameronmcefee.com/
it really a shame that only video and image is the only file extension supported by this forum
i think i miss something :!!!
http://www.lh-consultant.net/adobe/guiguide960grid.psd

Answer 1

my default setting in ps was when post
Edit  ->preference -> General->option->Snap vector tools and transforms to pixel grid [enable|check]
A) i change after post this setting preference for Gridline display  every 1px (& 1 subdivision)
I don't know if doing this is the way to go  !!
B) i don't see in ps  preference a label call  "align to pixel grid"  !!
any clue ?
C) when re open the psd file and zoom /de zoom the width value  is ok  !
very strange ,  in the past after try to "reverse " css to photoshop  i find that pixel perfect is a myth !!
perhaps the problem that occur here ll help to set ps for pixel perfect design !!
regard's
http://blogs.adobe.com/jkost/2012/05/align-to-pixel-grid-in-photoshop-cs6.html
it is a shame to not be able to edit old post content but only the last one !!