System.Diagnostics.EventLog doesn't contain correct message

As mentioned in the comments, Get-WinEvent uses the EventLogReader class to enumerate the events queried, and then calls EventRecord.FormatDescription() on each resulting record to render the localized message.

Here's a sample console application to fetch and print the rendered message of each of the first 10 Warning (Level=3) events in the Application log:

using System;
using System.Diagnostics.Eventing.Reader;

class Program
{
    static void Main(string[] args)
    {
        // construct an EventLogQuery object from a log path + xpath query
        var xpath = "*[System[Level=3]]";
        var query = new EventLogQuery("Application", PathType.LogName, xpath);

        // instantiate an EventLogReader over the query
        var reader = new EventLogReader(query);

        // read the events one by one
        var counter = 0;
        EventRecord record = null;
        while ((record = reader.ReadEvent()) is EventRecord && ++counter <= 10)
        {
            // call FormatDescription() to render the message in accordance with your computers locale settings
            var renderedMessage = record.FormatDescription();
            Console.WriteLine(renderedMessage);
        }
    }
}

Beware that it's entirely possible for FormatDescription() to return an empty string - this will occur when the event logging provider didn't provide a message template for the given event id.