The anomalies of hmac_sha1 performance testing on Android

I am testing the performance of cryptographic libs on Android 4.03. But the result shows an anomalies like the figure blow: Peformance testing of hmac_sha1 on Android

I gradually increase the input size, the execution time increases in a linear way. But when the size reaches a point (197 * 160B in this figure). The performance suddenly becomes better. The same thing happens in testing AES_CBC. So I want to ask why it happens in this way?