Digital color is not defined only by RGB numbers. A color profile describes how those numbers relate to actual colors. sRGB is widely used because it is a common reference space for web and general-purpose display.
Why colors sometimes shift
If an image was created in a wider-gamut color space and an application ignores or mishandles its profile, colors can appear too saturated, too dull or otherwise different. Conversion software may preserve, transform or discard profile information depending on its configuration.
Why sRGB is common on the web
sRGB offers predictable interoperability across a broad range of devices and software. That does not mean it can represent every color a modern display can show, but it remains a practical baseline for general web content.
What to check after conversion
For color-critical work, inspect the output in a color-managed application and confirm the embedded profile. Do not judge only by one browser window or assume file format alone determines color accuracy.