When I use my 300mm F4 MF-only lens/TAMRON 2X TC/PENTAX *ist D, I am somehow able to take pictures at a maximum aperture of F1.7! Also F2.8, F4, F5.6 etc.. This lens supposedly has a maximum aperture of F4, and when used on its own (no 2X TC), F4 is as far as I am able to "open up".

Full name of 300mm lens: PENTAX SMC PENTAX-A* F4 300mm.
Full name of 2X TC: 2X Pz-AF BBAR MC7 TAMRON-F AF Tele-Converter.

I have not experienced this issue when using the TC with my other two lenses (28-80mm F3.5-4.5 and 100mm F2.8), only with the 300mm combined with the TC. Attached is a sample photo with EXIF data showing an aperture value of F1.7.

Attachment 63285
Excuse the quality, it is just a "grab shot" for an EXIF data sample. For those of you without an EXIF reader handy, or somehow find the data unreadable, click here, then scroll down a bit.

Code:
Camera Brand: PENTAX Corporation 
Camera Model: PENTAX *ist D      
Exposure Time: 1/400 sec.
F-number: F1.7
ISO Speed Rating: 1600
Flash Fired: Flash did not fire, compulsory flash mode.
Metering Mode: Center-Weighted Average
Exposure Program: Aperture priority
Focal Length: 0.0 mm
Contrast: Normal
Saturation: Normal
Sharpness: Normal
Any insight as to why this f-number-issue would be great...