ascii is a subset of utf-8. All symbols in ascii are present in utf8 with the same value.
But utf-8 is a multi-byte encoding, so a symbol may consist of 1, 2 or even 4 bytes, so obviously ascii doesn't contain all symbols utf-8 does.
Now, more on topic: What "Der Meister" said is absolutely correct. Windows uses utf-16 internally, therefore its support for utf8 is limited. Also, it expects a BOM (Byte order mark) at the beginning of a unicode-file. All other textfiles are assumed to be (extended) ascii.
Unix on the other hand doesn't expect a BOM, so the first 2 bytes are interpreted as symbols.
You have 2 choices: Either stick to ASCII like "Der Meister" suggested, or write a (very simple) program to quickly add or remove the Signature (0xFEBBBF) to/from files.