AVAssetExportSession frame discrepancy after export

I'm seeing an oddity with audio files created via AVAssetExportSession and AVAssetExportPresetAppleM4A.

In every case the exported m4a files report a different frame length than the number frames that can actually be read into an AVAudioPCMBuffer for playback.

Below is a simple command line app demonstrating the issue.

In my test case the output was:
=================================
Audio File: export.m4a
File frame length: 6302184
Buffer frame length: 6301632
Frame length discrepancy: 552
=================================

I've replicated this issue on iOS and macOS. It appears to be fundamental to AVAssetExportSession on all platforms or at least to my pattern of usage.

Can anyone tell me if I'm doing something wrong? Is this possibly a bug in AVAssetExportSession?

Thanks!

Code Block Swift
import Foundation
import MediaPlayer
//Requires file path to an audio file
guard CommandLine.arguments.count == 2 else {
		print("Usage: \(CommandLine.arguments[0].split(separator: "/").last!) <filepath>")
		exit(1)
}
var sourceFile = CommandLine.arguments[1]
let sourceURL = URL(fileURLWithPath: sourceFile)
var destinationURL = sourceURL.deletingLastPathComponent()
destinationURL.appendPathComponent("export.m4a")
let exportGroup = DispatchGroup()
exportGroup.enter()
exportAsset(from: sourceURL, to: destinationURL) {
		printFrameCount(url: destinationURL)
		exportGroup.leave()
}
exportGroup.wait()
exit(0)
func exportAsset(from: URL, to: URL, completion: @escaping ()->Swift.Void) {
		let asset = AVURLAsset(url: from,
													 options: [AVURLAssetPreferPreciseDurationAndTimingKey: true])
		guard let exporter = AVAssetExportSession(asset: asset,
																							presetName: AVAssetExportPresetAppleM4A) else
		{
				fatalError("unable to create AVAssetExportSession")
		}
		exporter.shouldOptimizeForNetworkUse = false;
		exporter.outputFileType = AVFileType.m4a
		
		exporter.outputURL = to
		
		//delete target file if it exists
		if FileManager.default.fileExists(atPath: to.path) {
				try? FileManager.default.removeItem(at: to)
		}
		exporter.exportAsynchronously {
				guard exporter.error == nil else {
						fatalError("AVAssetExportSession error: \(String(describing: exporter.error))")
				}
				completion()
		}
}
func printFrameCount(url: URL) {
		do {
				let file = try AVAudioFile(forReading: url)
				let fileLength = file.length
				
				guard let buffer = AVAudioPCMBuffer( pcmFormat: file.processingFormat,
																						 frameCapacity: AVAudioFrameCount(fileLength) ) else {
						fatalError("AVAudioPCMBuffer allocation failed")
				}
				
				try file.read(into: buffer)
				let bufferLength = buffer.frameLength
				print("=================================",
						"Audio File: \(url.lastPathComponent)",
						"File frame length: \(fileLength)",
						"Buffer frame length: \(bufferLength)",
						"Frame length discrepancy: \(fileLength - Int64(bufferLength))",
						"=================================",
						separator: "\n")
		} catch {
				fatalError("AVAudioFile error: \(String(describing: error))")
		}
}