Fix inexact and inconsistent frame seek

[bzczb]

Fixes #2115 seeking by frame is inexact and inconsistent

• [x] I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• [ ] I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• [x] I have properly documented new or changed features in the documentation or in the docstrings
• [x] I have properly explained unusual or unexpected code in the comments around it