Looking into Google webmaster tools it happened to me many times to see for some of my sites these type of inconsistency, in the text they say my site is faster than 63% of sites, but when I look at the graph the site is on the SLOW background (over the 1.5 seconds line).
So do you know if my site is considered fast or slow by Google?
Is it me not able to read properly the graph?
Being faster then 63% of sites doesn't mean your site is fast. It just means you're faster then 63% of the other websites in the world.
Also, fast is a relative term. Google apparently defines fast as being approximately 1 second or faster. So to Google your site is slow. But to humans your site may still be seen as fast.
Based on the chart I see I think you have room for improvement on your page loading speed. At the same time, your site looks fast enough that page speed won't hurt your pages' rankings. That is reserved for sites that load very slow. Yours is definitely not that slow.
It's slow, there are lots of even slower ones. Fast does not mean "in the top X%", it means "average load time is less than 1.5 seconds".
In that paragraph it mentioned the "20th percentile". This is the green area labelled "fast", and it's the top 20% of all sites. If your site is faster than 61% of sites, you are in the top 40%. You would need to be faster than 80% of sites to be in the 20th percentile.
I think your main problem is the large variation in loading times. The sites I have in GWT are fairly consistent (although gradually increasing in some cases).
