With any type of data, there is an inherent risk of misinterpretation. My advice to elucidators is to consider multiple solutions and examine each one thoroughly. In the end, the answer to any problem set lies in tying together the bits of information in hopes of understanding the bigger picture.
Recap of the problem: The ESI+ MS shows a single [M+H]+ at m/z 102 allowing a maximum carbon count of 8. The 13C NMR shows there to be 12 carbons. How can the data from the MS and NMR present such different results for the same unknown?
The data from both the 13C NMR and DEPT-135 spectra are consistent with a mixture of two similar compounds at approximately a 1:1 ratio.
A mixture with an ESI+ MS exhibiting a single molecular ion indicates that the compounds in the mixture differ by a proton. For example, the mixture comprises of one compound with an R-NH2 group and the other with an R-NH3+ group. Some example amine/aminium mixtures are shown below.