the Garrard 301 plater is concaved.

The original Well Tempered platter was also concave, intentionally by design.

Most platters are slightly concave. Leveling them is easy as long as you align the level on a tangent to the outside edge. Of course you should be using a nice long level anyways!

I usually use both a long and a short I just got a new bubble and was trying it out. Didn't know the Garrard plater was concave always figured it was flat seems I was wrong haha. 

more funny then anything how we can make a simple mistake and have your whole set up off with out realising it. 

My early grease bearing 301 has a slightly concave platter too. I believe it is common. I believe it was made this way intentionally. If it bothers you, there are after-market platters on the market that offer slight benefits that may or may not be audible depending on the rest of your deck and system. Check out Peak HiFi in the UK. I bought Shaun's replacement brass bearing even though my original bearing looked brand new. I am a big believer in having the stoutest most precise main bearing possible. https://www.peakhifi.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=718
https://www.peakhifi.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=96

After noting the comments of the "slightly concave platter"
I proceeded to check my mid run 301 grease bearing 
and low and behold it is slightly concave.

There's always something to learn in this hobby.