I have subscribed for a broadband plan for 2Mbps. However there is considerable inconsistencies between the promised speed and the actual download speeds. For instance the maximum download speeds I get is about 300Kbps. Why this disparity? Is the speeds lost to attenuation or is the 2Mbps a shared line accessed by multiple subscribers?

possible duplicate of Why is my speed only limited to 12% of my max bandwidth? – Ƭᴇcʜιᴇ007 Jan 30 '12 at 20:21
Line speeds are measured in decimal bits per second. Data speeds are measured in binary bytes per second. The download speed you are getting is 300KB/s, not Kbps. Let's do the math:
2Mbps (line speed) = 2,000,000 bits per second
2,000,000 bits per second = 2,000,000/8 bytes per second = 250,000 bytes per second
250,000 bytes per second / 1024 bytes per KiB = 244KB/s (data rate)
The line has to carry control and address information as well as data. Typically, the peak true data rate is about 95% of the line rate, so you'd expect 244*.95 or 232KB/s to be the peak data rate for a line with a 2Mbps line rate.
You are almost certainly mixing bits and bytes. 300 kB/s (kilobytes per second) equals 2.4 Mb/s (Megabits per second), or slightly more than your stated plan bandwidth.