Problems extracting csv from rosbag

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Hello, I am extracting the data of a rosbag to a csv file. The problem is that I do not want to mix strings with numbers so I first extract the numbers with 'rostopic echo -p --nostr -b bag.bag > bag_nostr.csv' and then I extract the specific topic containg string doing 'rostopic echo -p -b bag.bag/string > bag_str.csv'. After that, if I compare the files, both have the same number of rows (aka messages) but the time stamped in each message is different from the one in the same row of the other csv!!! How can that happend? How can I solve it?

Thanks

Cheers!