[plt-scheme] continuation marks

Why does the output differ for these two cases:

 mzscheme -t
test.ss                                                                         
Welcome to MzScheme v372 [3m], Copyright (c) 2004-2007 PLT Scheme Inc.
(4 2)
(4)

John Leuner

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(module |test| mzscheme
	(require (lib "control.ss"))

	(with-continuation-mark 'b 2
	     (list (with-continuation-mark 'b 4
			(print
			 (continuation-mark-set->list (current-continuation-marks) 'b)))))

	(newline)
	(with-continuation-mark 'b 2
	     (with-continuation-mark 'b 4
			(print
			 (continuation-mark-set->list (current-continuation-marks) 'b))))

	;output is
	;(4 2)
	;(4)
	)