did you check that the offset and limit values are correct. This can be done by adding a line after the let:
{$var_offset}
This should print the value. To get the integer value you need to ask the the $node.object.data_map.offset.content. So by changing the code as shown below should work:
Without the .content, you'll get the object attribute itself (an instance of eZContentObjectAttribute).
With the .content, you'll get the object attribute's content. What this exactly is, depends on the used datatype.
For a text line for example, you'll get a variable with the text that was filled in. For an integer, you'll get a variable with the integer that was filled in. For a binary file, you'll get an instance of eZBinaryFile. Etc.