Halo coronal mass ejections ( CMEs ) were found to be significantly faster than normal CMEs , which was a long-standing puzzle . In order to solve the puzzle , we first investigate the observed properties of 31 limb CMEs that display clearly loop-shaped frontal loops . The observational results show a strong tendency that slower CMEs are weaker in the white-light intensity . Then , we perform a Monte Carlo simulation of 20 000 artificial limb CMEs that have average velocity of \sim 523 km s ^ { -1 } . The Thomson scattering of these events is calculated when they are assumed to be observed as limb and halo events , respectively . It is found that the white-light intensity of many slow CMEs becomes remarkably reduced as they turn from being viewed as a limb event to as a halo event . When the intensity is below the background solar wind fluctuation , it is assumed that they would be missed by coronagraphs . The average velocity of “ detectable ” halo CMEs is \sim 922 km s ^ { -1 } , very close to the observed value . It also indicates that wider events are more likely to be recorded . The results soundly suggest that the higher average velocity of halo CMEs is due to that a majority of slow events and a part of narrow fast events carrying less material are so faint that they are blended with the solar wind fluctuations , and therefore are not observed .