RSpec #should_recieve can't count correctly with threads

thread_safe_expectation_spec.rb

require 'rspec'

T = 10000
N = 200

class ConcurrentSender
  def initialize(collaborator,message,threads,sends)
    @collaborator,@message,@threads,@sends = collaborator,message,threads,sends
    @tasks = []
  end

  def run
    @threads.times do
      @tasks << Thread.new do
        @sends.times { @collaborator.send @message }
      end
    end
    @tasks.each{|t| t.join }
    @tasks = []
  end
end

describe "should_recieve expectation counter" do
  it "should correctly count messages in concurrent execution" do
    dummy = Object.new
    dummy.should_receive(:foo).exactly(N*T).times
    cs = ConcurrentSender.new(dummy,:foo,T,N)
    cs.run
  end
end

$ ruby -v
ruby 1.8.7 (2010-01-10 patchlevel 249) [x86_64-linux]
$ rspec
F

Failures:

  1) should_recieve expectation counter should correctly count messages in concurrent execution
     Failure/Error: dummy.should_receive(:foo).exactly(N*T).times
       (#<Object:0x7fba905a24e8>).foo(any args)
           expected: 2000000 times
           received: 1999800 times
     # ./spec/thread_safe_expectation_spec.rb:26

Finished in 43.46 seconds
1 example, 1 failure

$ rbenv global jruby-1.5.6 
$ jruby -v
jruby 1.5.6 (ruby 1.8.7 patchlevel 249) (2010-12-03 9cf97c3) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_26) [amd64-java]
$ jruby -S rspec
F

Failures:

  1) should_recieve expectation counter should correctly count messages in concurrent execution
     Failure/Error: dummy.should_receive(:foo).exactly(N*T).times
       (#<Object:0x4556a792>).foo(any args)
           expected: 2000000 times
           received: 1923606 times
     # ./spec/thread_safe_expectation_spec.rb:26
     # :1

Finished in 12.45 seconds
1 example, 1 failure

$ rbenv global ruby-1.9.2 
$ ruby -v
ruby 1.9.2p318 (2012-02-14 revision 34678) [x86_64-linux]
$ rspec
.

Finished in 19.45 seconds
1 example, 0 failures