How can the computed width style deviates so much in internet explorer?

HTML & CSS
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1

I have a class to assign width of a table such as:
.FormField { width:120pt; vertical-align:top; }

It works fine in chrome and firefox as the computed width is width: 160px; but in explorer it’s width: 324.01px;. How can it deviates so much? Can somehow please tell me why and how can i solve it? Thank you in advance.