Java performance anomaly: the loop with 'beta' method is 2x slower than loop with 'alpha' even though the methods are identical.

Main.java

import java.lang.reflect.Field;
import java.nio.ByteBuffer;

public class Main {
	private static final int N = 1_000_000;
	private static Field stringBytes;
	private static String testString;

	static {
		// setup test string:
		StringBuilder builder = new StringBuilder(1024);
		for (int i = 0; i < 100; i++) {
			builder.append("Hello, world!");
		}
		testString = builder.toString();
		// setup String byte array field:
		try {
			Field stringValue = String.class.getDeclaredField("value");
			stringValue.setAccessible(true);
			stringBytes = stringValue;
		} catch (NoSuchFieldException e) {
			e.printStackTrace();
		}
	}

	public static void main(String[] args) throws IllegalAccessException {
		ByteBuffer expectedBuffer = ByteBuffer.allocate(2000)
			.put(testString.getBytes());
		ByteBuffer testBuffer = ByteBuffer.allocate(2000);
		//
		for (int i = 0; i < 10; i++) {
			runTests(testBuffer, expectedBuffer);
		}
	}

	private static void runTests(ByteBuffer testBuffer, ByteBuffer expectedBuffer) throws
		IllegalAccessException {
		long startTime = System.nanoTime();
		for (int i = 0; i < N; i++) {
			testBuffer.clear();
			slow(testBuffer);
			checkResult(testBuffer, expectedBuffer);
		}
		long endTime = System.nanoTime();
		System.out.println("Slow: \t" + (endTime - startTime));
		//
		startTime = System.nanoTime();
		for (int i = 0; i < N; i++) {
			testBuffer.clear();
			alpha(testBuffer);
			checkResult(testBuffer, expectedBuffer);
		}
		endTime = System.nanoTime();
		System.out.println("Alpha: \t" + (endTime - startTime));
		//
		startTime = System.nanoTime();
		for (int i = 0; i < N; i++) {
			testBuffer.clear();
			beta(testBuffer);
			checkResult(testBuffer, expectedBuffer);
		}
		endTime = System.nanoTime();
		System.out.println("Beta: \t" + (endTime - startTime));
	}

	// make sure the JIT doesn't discard computations
	private static void checkResult(ByteBuffer got, ByteBuffer expected) {
		if (!got.equals(expected)) {
			throw new RuntimeException(
				String.format("expected: %s, got %s", expected, got)
			);
		}
	}

	private static void slow(ByteBuffer source) {
		source.put(testString.getBytes());
	}

	private static void alpha(ByteBuffer source) throws IllegalAccessException {
		source.put((byte[]) stringBytes.get(testString));
	}

	private static void beta(ByteBuffer source) throws IllegalAccessException {
		source.put((byte[]) stringBytes.get(testString));
	}
}