Its about compression as I said, if you read the following its like listing to Hussain Obama, you have to read between the lines and understand the system and how it works!

The VIXIA HG21 boasts 1920 x 1080 Full HD, and has a CODEC capable of recording 24Mbps -- the highest AVCHD bitrate --through Canon's own Full HD CMOS Image Sensor and advanced DIGIC DV II image processor. Other exclusives are a new Genuine Canon 12x HD video lens, SuperRange Optical Image Stabilization and Instant AutoFocus and a host of other advanced features including an HDMI™ Connector. Enhanced and simplified controls give you more choices and easier operation. Remarkable image reproduction and advanced features are yours in this compact AVCHD Format Hard Disk Drive camcorder which easily goes along with you, wherever you are, to capture those unforgettable moments in life- in true HD.

AVCHD is not true HD, but the camera is capable of recording or "Seeing" true HD the Hard drive is not big enough to store true uncompressed HD.