Why CO2 is not reducing the pH of NaOH as the way it is expected
There are a so-called acid error and an alkaline error with glass electrodes. Which means that the pH electrode shows a systematic bias when your solution is too acidic or too basic and if it contains lot of sodium ions. The value which you quote for +13 are unreliable. So if you are bubbling carbon dioxide into NaOH solution, you are indeed consuming NaOH and converting it into sodium carbonate. The Wikipedia article on glass electrode is poorly written however if you search the term "alkaline error of glass electrodes" you will hundreds of better results. 

May be you should follow the pH changes spectrophotometrically using a proper indicator (dyes) in NaOH.

Also keep in mind that you shouldn't store NaOH in glass containers because glass starts to dissolve in NaOH (very slow process though)