Home Office to keep DNA of innocent for six years, not 12

Home Office ministers are expected to announce a six-year limit on the retention of DNA profiles on the national database for people arrested but not convicted of any offence, bringing it more in line with Scotland, where five years is the norm.

The case for keeping DNA profiles of suspects on record for up to 12 years was undermined when the Jill Dando Institute for Crime Science, whose research the Home Office used to justify the proposals, said its work was unfinished and should not have been relied upon. The Home Office claims that six years covers the likely period in which someone might offend after having their DNA taken.

Am I missing something here or this an entirely arbitrary statement? Why is six years the 'likely period' to offend? Why should an innocent person on the database be any more likely to offend than an innocent person not on the database? Surely the bigger question is why the DNA of innocent people should be stored at all - six years is no better than twelve imo.