Why the psudotime of my root cell is not zero?

[wanglu2014]

cds <- order_cells(cds,root_cells = 'cell1') The expression matrix in this toy example was a 150cell*7gene full matrix with number. This example is crucial for our study, which can not be enlarged. How can I make sure psudotime of my root cell is zero, and psudotime start from it.