A strange effect with pitch bend

I want to make a pitch bend (portamento) that lowers the tone from A to G, i.e. a double half tone. I use my PC keyboard to create the pitch bend. I press the H key and then the 1 key and then I depress the H key. I do this in a separate track and then edit the result into the track in the position where I want the pitch bend to be. However, I have observed the following when I play that track:

- The pitch bend lowers the tone by only one half tone.

- However, I can make the pitch bend lower the tone a double half tone by opening any menu in Garageband using the left mouse key and just keeping that menu open for as long as the pitch bend lasts.

Does anyone understand what's going on?

I have Garageband '09 and I've just started using it.