The first 128 characters of ANSI and UTF-8 are the same, with the same encoding. If the files you create with Aptana Studio 3 only have these characters, Notepad++ isn't going to be able to tell the difference. An ANSI file and a UTF-8 file are going to appear identical.
You can observe the same behaviour using only Notepad++
If you save a file in Notepad++, that only has characters in the 0-127 range, with the encoding
UTF-8 without BOM. Then re-open the file in Notepad++ it will show the encoding as
If you're wondering why
UTF-8 without BOM and not
UTF-8 that is because the Unicode Standard states:
"Use of a BOM is neither required nor recommended for UTF-8" Which is what I imagine Aptana Studio 3 follows when it saves files with the encoding UTF-8.