What makes same wattage led light bulb brighter than the other?
The biggest factor in light bulbs is the transmittance coefficient which is the amount of light that gets through the diffused cover.  

I asked Klus for the transmittance coefficient of three of their LED diffusers.  A clear smooth polycarbonate cover has a transmittance coefficient of 96%. 

This is the reply from Klus for various diffusers.  The diffuser transmittance coefficient can account for 20% difference from one bulb to another.

(HS)1369- 70% (frosted / opal)
(LIGER)17031-55%  (transparent with frosted matte finish)
(HS)17111- 30%  (Satin light focusing cover) 

Image of Light Focusing Diffuser:
Thickness is a big factor in transmittance. 







The loss in efficiency of the LED drivers can range from 5% to 30%. 





A high thermal heat factor can reduce the luminosity by about 10%. 

The top image is a high power Lumiled Rebel ES and the bottom a mid power Samsung LM301B, both high efficacy LEDs. 









The efficacy (lumens/watt) of the LED can vary significantly from less than 100 lm/W to 220 lm/watt. 

A 5700K 70 CRI will have a much higher efficacy (200 lm/W) than a 2700K 90 CRI (150 lm/W) within the same LED product line.  

The lower (warmer) the color temperature and the higher the CRI  (more like sunlight) both will lower the lumens.  This is due to the amount of red. 

All white LEDs are a deep blue (450 nm) LED that uses phosphor wavelength converter to convert the blue wavelength to green and red.  The phosphor conversion gets less efficient as the wavelength rises from blue to red.  








Photopic Luminous Efficacy (Relative Sensitivity Curve
for the C.I.E. Standard Observer) of 555 nm green is 100,000, 525 nm red: 32,300, and 450 nm blue: 3,800.  

To get the photopic luminous efficiency the brightness of the 555 nm source was reduced until the observer felt that the two sources were equal in brightness. The fraction by which the 555 nm source was reduced, became the luminous sensitivity with respect to the second observed wavelength. 

1 watt of deep blue radiant flux has a luminosity factor (conversion from radiometric (radiant watts) of  555 nm of green has factor of 683 (100%), and 625 nm red has a factor of 220 (32%), and 450 nm blue 26 (3%).   

Comparison of irradiance spectral distribution in Lumens and Watts for the same LED: