Different people interpreted differently. To say that interpolations changed from the late 18th century to the present day would be an incorrect statement. The popularity of interpenetration from the late 18th century to the present day has changed.
The interpretation in question is if negroid people are people. Some of the very first abolitionist, owned slaves, like Thomas Jefferson and George Washington. The question "Why didn't they just free their slaves?" requires an essay in of it's self, but the short answer is that if it was financially possible, it would only mean transferring from one plantation to another, or otherwise risk putting them in inhumane conditions. Alexander Hamilton, was an abolitionist his whole life, he never inherited a plantation or any slaves, so he never had to deal with this issue.
Not every founding father or more importantly representative had this view. When Thomas Jefferson first wrote the Decoration of Independence, the grievance whereas it negativity talked about slavery was removed as a compromise so that the thirteen colonies could vote Yea. It should also be noted that racism and slavery are two separate issues. In many cases through out the 18th century, it is more common to find negroid people selling other negroid people to slavery than of caucasoid people doing the same.
In July of 1776, in Carpenters Hall, the main issue that was facing the Country was Independence. Slavery and the freedom of negroid people will have to be solved another date if the Nation where to ever live to see the day. The Nation could not afford to go to war among itself, crash it's own economy, and fight for independence in the same go. This fight would be solved in the Civil War.
Thus the Answer: The interpretations of the fact that all men are created equal did not change from the writing of the Decoration of Independence, but the popularity and realization of race being a negligent factor in humanity, and the evils of slavery, established its self in American Culture.
