Sparse Iterative Solvers Scalability Issue in MAGMA Library

I have executed some tests using hepta diagonal sparse matrices on conjugate gradient implementations in MAGMA library. Regarding scalability, we expect execution time, ET(N), to be proportional to O(14N) i.e. the overall number of non-zero elements. Thus, if we use hepta with 2N, we expent ET to grow as O(2*14N) thus just doubling. But the results below for MAGMA CSR shows: N growing from 1 to 30 but ET is growing from 12 to 18 (from 1 to 1.5). Can you please clarify/suggest why the execution time is not scaling as expected?