When you output items from a PowerShell function (even with the return statement), it will enumerate collections and send them down the output stream one at a time. That's usually what you want to happen, but sometimes that's not the case.
In PowerShell v3 or later, you can use this:
Write-Output -NoEnumerate $a
Which is nice and clear. For v2 compatibility, you need to use the unary comma operator. This wraps your array in another array, so when PowerShell enumerates the outer array, it still sends your inner one intact down the pipeline:
Much less clear in terms of code readability, but it's all you've got prior to v3. (It's also quite a bit faster than calling Write-Output, if you have performance hotspots that need tuning at some point.)