CRM 2011 field On Change Event Javascript getting Value using XRM.Page.getAttribuites(fieldName).getValue()

Hi I am experiencing a strange behaviour when I try to get a field value Xrm.Page.getAttribute(totalAmountFieldName).getValue(); on change. It gives the previous value not the current changed value.

but If i use  window.crmForm.all.totalAmountFieldName.DataValue; this gives the correct change value.

Ex :

Total Amount = 100

Changed to 200 and the Onchange event fires I get 100 from Xrm.Page.getAttribute(totalAmountFieldName).getValue(); But window.crmForm.all.totalAmountFieldName.DataValue gives the correct value 200.

Why is it behaving different? Can some one help.