IE (PC) last line in a paragraph glitch

I assume this is a very basic question but I've never seen it 
addressed anywhere...

On a PC using IE4+ the last line in a paragraph has always got extra 
space between it and the preceding line. I've seen this on so many 
sites and for so long that I just accept it without question but for 
the second time I've had a client insist that it's not acceptable. I 
really hate telling someone that I don't know why something occurs 
but they'll just have to trust me and live with it. Can anyone speak 
to what causes this phenomenon?

(I know that I can sometimes avoid the problem by using two <br> tags 
but there are many cases where that is not possible or practical.)

TIA

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