What makes V1 one or two knots less than Vr on this performance table? (B738 AFM, 0-2000 feet elevation, up to 27°C) Example of V1 and Vr differences.

The table shows that the V1 speeds are invariably one or two knots lower than their matching Vr speeds. Given that V1 can equal Vr, what's the limiting factor (probably factors) that puts V1 one or two knots below Vr? Vmbe might be one, but is it the only possible limiting factor here? 

Or perhaps the V1 speeds are the same with the matching Vmcg speeds because that's operationally more advantageous in some ways than V1s being the same as the higher Vr speeds? 

Or are the V1 speeds the speeds that bring the balanced field lengths, which require the mimimum runway distance?
$V_R$ is:

FAR 23 (e.g. multi-engine small planes):


  $\geq 1.05\;V_{MC}$ (105% of minimum control in flight)
  
  $\geq 1.1\;V_{S_1}$ (110% of V-stall for a specific configuration)


FAR 25 (e.g. transport jets):


  $\geq V_1$
  
  $\geq 1.05\;V_{MC}$
  
  Must result in $V_2$ before 35 ft height
  
  Must result in $V_{LOF}$ [when the airplane is rotated at its maximum practicable rate]
  
  
    $\geq 1.1\;V_{MU}$ (all engines operative; AEO)
    
    $\geq 1.05\;V_{MU}$ (one engine out)
    
    $\geq 1.08\;V_{MU}$ (AEO, geometry limited)
  


Geometry limited is when there is a greater risk of a tail-strike, e.g., a stretched variant of an airplane (think A321 or 737-900).

So the 1-2 knot disparity in your example can be down to the difference between $V_{MCG}$ and the 105% of $V_{MC}$ or the different $V_{MU}$ speeds. (See below for full text on points 2 and 4.)

To clear any confusion about the Weight:V1 tables such as the one in the question, those tables are not for 1 runway length, those weights are the field limit weight, i.e., each higher weight in the table is a longer runway.

(Twitter) $V_{MU}$ test.



Source: http://wpage.unina.it/fabrnico/DIDATTICA/SPER_VOLO/APPUNTI_3CFU/11.2-TO&amp;LAND.pdf

FAA AC No: 25-7C text:


  
  VR may not be less than V1; however, it can be equal to V1 in some cases.
  VR may not be less than 105 percent of the air minimum control speed (VMCA).
  VR must be a speed that will allow the airplane to reach V2 at or before reaching a height of 35 ft. above the takeoff surface.
  VR must be a speed that will result in liftoff at a speed not less than 110 percent of VMU (unless geometry limited) for the all-engines-operating condition and not less than 105 percent of the VMU (unless geometry limited) determined at the thrust/weight ratio corresponding to the one-engine-inoperative condition for each set of conditions such as weight, altitude, temperature, and configuration when the airplane is rotated at its maximum practicable rate.
  


(Emphasis mine.)