Applying for a tourist visa to Guangzhou, do i need an Invitation Letter of Duly Authorized Unit?

I recently applied for a tourist visa 'L' to visit Guangzhou for a duration of 14 days. I was there in September last year but only spent 6days. My visa application was return with reason stating the i have no notification of visa. At the consular office, i was told that i need to get an invitation letter of duly authorized unit from China. I read more information about this on the internet but could only find that the invitation letter was only applicable to anyone applying for work, business, or study purpose in China. I tried to contact some travel agent through the internet to see if they could secure the invitation for me but all prove they do not provide such services.
On this page, it states that for the tourist visa category 'L', the invitation is not required. So, why was my application returned to go and get the invitation letter?